Epidemiology

Epidemiology

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Encyclopedia
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health
Public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals" . It is concerned with threats to health based on population health...

 research, and helps inform policy decisions and evidence-based medicine
Evidence-based medicine
Evidence-based medicine or evidence-based practice aims to apply the best available evidence gained from the scientific method to clinical decision making. It seeks to assess the strength of evidence of the risks and benefits of treatments and diagnostic tests...

 by identifying risk factor
Risk factor
In epidemiology, a risk factor is a variable associated with an increased risk of disease or infection. Sometimes, determinant is also used, being a variable associated with either increased or decreased risk.-Correlation vs causation:...

s for disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...

 and targets for preventive medicine
Preventive medicine
Preventive medicine or preventive care refers to measures taken to prevent diseases, rather than curing them or treating their symptoms...

. Epidemiologists are involved in the design of studies, collection and statistical analysis of data, and interpretation and dissemination of results (including peer review
Peer review
Peer review is a process of self-regulation by a profession or a process of evaluation involving qualified individuals within the relevant field. Peer review methods are employed to maintain standards, improve performance and provide credibility...

 and occasional systematic review
Systematic review
A systematic review is a literature review focused on a research question that tries to identify, appraise, select and synthesize all high quality research evidence relevant to that question. Systematic reviews of high-quality randomized controlled trials are crucial to evidence-based medicine...

). Major areas of epidemiological work include outbreak
Outbreak
Outbreak is a term used in epidemiology to describe an occurrence of disease greater than would otherwise be expected at a particular time and place. It may affect a small and localized group or impact upon thousands of people across an entire continent. Two linked cases of a rare infectious...

 investigation, disease surveillance and screening (medicine)
Screening (medicine)
Screening, in medicine, is a strategy used in a population to detect a disease in individuals without signs or symptoms of that disease. Unlike what generally happens in medicine, screening tests are performed on persons without any clinical sign of disease....

, biomonitoring
Biomonitoring
Aquatic biomonitoring is the science of inferring the ecological condition of rivers, lakes, streams, and wetlands by examining the organisms that live there...

, and comparisons of treatment effects such as in clinical trials. Epidemiologists rely on a number of other scientific disciplines such as biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 (to better understand disease processes), biostatistics
Biostatistics
Biostatistics is the application of statistics to a wide range of topics in biology...

 (to make efficient use of the data and draw appropriate conclusions), and exposure assessment
Exposure assessment
Exposure assessment is a branch of environmental science that focuses on the processes that take place at the interface between the environment containing the contaminant of interest and the organism being considered. These are the final steps in the path to release an environmental contaminant,...

 and social science disciplines (to better understand proximate and distal risk factors, and their measurement).

Etymology


Epidemiology, literally meaning "the study of what is upon the people", is derived , suggesting that it applies only to human populations. However, the term is widely used in studies of zoological populations (veterinary epidemiology), although the term 'epizoology
Epizoology
Epizoology, or veterinary epidemiology, is the study of disease patterns within animal populations. See epidemiology....

' is available, and it has also been applied to studies of plant populations (botanical epidemiology).

The distinction between 'epidemic' and 'endemic' was first drawn by Hippocrates
Hippocrates
Hippocrates of Cos or Hippokrates of Kos was an ancient Greek physician of the Age of Pericles , and is considered one of the most outstanding figures in the history of medicine...

, to distinguish between diseases that are 'visited upon' a population (epidemic) from those that 'reside within' a population (endemic). The term 'epidemiology' appears to have first been used to describe the study of epidemics in 1802 by the Spanish physician Villalba in Epidemiología Española. Epidemiologists also study the interaction of diseases in a population, a condition known as a syndemic
Syndemic
Syndemic refers to the aggregation of two or more diseases in a population in which there is some level of positive biological interaction that exacerbates the negative health effects of any or all of the diseases...

.

The term epidemiology is now widely applied to cover the description and causation of not only epidemic disease, but of disease in general, and even many non-disease health-related conditions, such as high blood pressure and obesity.

History


The Greek physician Hippocrates
Hippocrates
Hippocrates of Cos or Hippokrates of Kos was an ancient Greek physician of the Age of Pericles , and is considered one of the most outstanding figures in the history of medicine...

 has been called the father of epidemiology. He is the first person known to have examined the relationships between the occurrence of disease and environmental influences. He coined the terms endemic
Endemic (epidemiology)
In epidemiology, an infection is said to be endemic in a population when that infection is maintained in the population without the need for external inputs. For example, chickenpox is endemic in the UK, but malaria is not...

 (for diseases usually found in some places but not in others) and epidemic
Epidemic
In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...

(for diseases that are seen at some times but not others).

Epidemiology is defined as the study of distribution and determinants of health related states in populations and use of this study to address health related problems.
One of the earliest theories on the origin of disease was that it was primarily the fault of human luxury. This was expressed by philosophers such as Plato
Plato
Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

 and Rousseau, and social critics like Jonathan Swift
Jonathan Swift
Jonathan Swift was an Irish satirist, essayist, political pamphleteer , poet and cleric who became Dean of St...

.

In the middle of the 16th century, a doctor from Verona
Verona
Verona ; German Bern, Dietrichsbern or Welschbern) is a city in the Veneto, northern Italy, with approx. 265,000 inhabitants and one of the seven chef-lieus of the region. It is the second largest city municipality in the region and the third of North-Eastern Italy. The metropolitan area of Verona...

 named Girolamo Fracastoro
Girolamo Fracastoro
Girolamo Fracastoro was an Italian physician, poet, and scholar in mathematics, geography and astronomy. Fracastoro subscribed to the philosophy of atomism, and rejected appeals to hidden causes in scientific investigation....

 was the first to propose a theory that these very small, unseeable, particles that cause disease were alive. They were considered to be able to spread by air, multiply by themselves and to be destroyable by fire. In this way he refuted Galen
Galen
Aelius Galenus or Claudius Galenus , better known as Galen of Pergamon , was a prominent Roman physician, surgeon and philosopher...

's miasma theory
Miasma theory of disease
The miasma theory held that diseases such as cholera, chlamydia or the Black Death were caused by a miasma , a noxious form of "bad air"....

 (poison gas in sick people). In 1543 he wrote a book De contagione et contagiosis morbis, in which he was the first to promote personal and environmental hygiene
Hygiene
Hygiene refers to the set of practices perceived by a community to be associated with the preservation of health and healthy living. While in modern medical sciences there is a set of standards of hygiene recommended for different situations, what is considered hygienic or not can vary between...

 to prevent disease. The development of a sufficiently powerful microscope by Anton van Leeuwenhoek
Anton van Leeuwenhoek
Antonie Philips van Leeuwenhoek was a Dutch tradesman and scientist from Delft, Netherlands. He is commonly known as "the Father of Microbiology", and considered to be the first microbiologist...

 in 1675 provided visual evidence of living particles consistent with a germ theory of disease
Germ theory of disease
The germ theory of disease, also called the pathogenic theory of medicine, is a theory that proposes that microorganisms are the cause of many diseases...

.

John Graunt
John Graunt
John Graunt was one of the first demographers, though by profession he was a haberdasher. Born in London, the eldest of seven or eight children of Henry and Mary Graunt. His father was a draper who had moved to London from Hampshire...

, a professional haberdasher
Haberdasher
A haberdasher is a person who sells small articles for sewing, such as buttons, ribbons, zips, and other notions. In American English, haberdasher is another term for a men's outfitter. A haberdasher's shop or the items sold therein are called haberdashery.-Origin and use:The word appears in...

 and serious amateur scientist, published Natural and Political Observations ... upon the Bills of Mortality in 1662. In it, he used analysis of the mortality rolls in London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 before the Great Plague
Great Plague of London
The Great Plague was a massive outbreak of disease in the Kingdom of England that killed an estimated 100,000 people, 20% of London's population. The disease is identified as bubonic plague, an infection by the bacterium Yersinia pestis, transmitted through a flea vector...

 to present one of the first life tables and report time trends for many diseases, new and old. He provided statistical evidence for many theories on disease, and also refuted many widespread ideas on them.

Dr. John Snow
John Snow (physician)
John Snow was an English physician and a leader in the adoption of anaesthesia and medical hygiene. He is considered to be one of the fathers of epidemiology, because of his work in tracing the source of a cholera outbreak in Soho, England, in 1854.-Early life and education:Snow was born 15 March...

 is famous for his investigations into the causes of the 19th century cholera epidemics. He began with noticing the significantly higher death rates in two areas supplied by Southwark Company. His identification of the Broad Street
Broadwick Street
Broadwick Street is a street in Soho, City of Westminster, London. It runs for 0.18 mile approximately west-east between Marshall Street and Wardour Street, crossing Berwick Street....

 pump as the cause of the Soho epidemic is considered the classic example of epidemiology. He used chlorine in an attempt to clean the water and had the handle removed, thus ending the outbreak. This has been perceived as a major event in the history of public health
Public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals" . It is concerned with threats to health based on population health...

 and can be regarded as the founding event of the science of epidemiology.

Other pioneers include Danish physician Peter Anton Schleisner, who in 1849 related his work on the prevention of the epidemic of neonatal tetanus on the Vestmanna Islands in Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...

. Another important pioneer was Hungarian
Hungary
Hungary , officially the Republic of Hungary , is a landlocked country in Central Europe. It is situated in the Carpathian Basin and is bordered by Slovakia to the north, Ukraine and Romania to the east, Serbia and Croatia to the south, Slovenia to the southwest and Austria to the west. The...

 physician Ignaz Semmelweis
Ignaz Semmelweis
Ignaz Philipp Semmelweis was a Hungarian physician now known as an early pioneer of antiseptic procedures. Described as the "savior of mothers", Semmelweis discovered that the incidence of puerperal fever could be drastically cut by the use of hand disinfection in obstetrical clinics...

, who in 1847 brought down infant mortality at a Vienna hospital by instituting a disinfection procedure. His findings were published in 1850, but his work was ill received by his colleagues, who discontinued the procedure. Disinfection did not become widely practiced until British surgeon Joseph Lister
Joseph Lister, 1st Baron Lister
Joseph Lister, 1st Baron Lister OM, FRS, PC , known as Sir Joseph Lister, Bt., between 1883 and 1897, was a British surgeon and a pioneer of antiseptic surgery, who promoted the idea of sterile surgery while working at the Glasgow Royal Infirmary...

 'discovered' antiseptics in 1865 in light of the work of Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

.

In the early 20th century, mathematical methods were introduced into epidemiology by Ronald Ross
Ronald Ross
Sir Ronald Ross KCB FRS was a British doctor who received the Nobel Prize for Physiology or Medicine in 1902 for his work on malaria. He was the first Indian-born person to win a Nobel Prize...

, Anderson Gray McKendrick
Anderson Gray McKendrick
Anderson Gray McKendrick was a Scottish physician and epidemiologist pioneered the use of mathematical methods in epidemiology...

 and others.

Another breakthrough was the 1954 publication of the results of a British Doctors Study
British Doctors Study
The British Doctors Study is the generally accepted name of a prospective cohort study which ran from 1951 to 2001, and in 1956 provided convincing statistical proof that tobacco smoking increased the risk of lung cancer.-Context:...

, led by Richard Doll
Richard Doll
Sir William Richard Shaboe Doll CH OBE FRS was a British physiologist who became the foremost epidemiologist of the 20th century, turning the subject into a rigorous science. He was a pioneer in research linking smoking to health problems...

 and Austin Bradford Hill
Austin Bradford Hill
Sir Austin Bradford Hill FRS , English epidemiologist and statistician, pioneered the randomized clinical trial and, together with Richard Doll, was the first to demonstrate the connection between cigarette smoking and lung cancer...

, which lent very strong statistical support to the suspicion that tobacco smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...

 was linked to lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...

.
  • History of emerging infectious diseases
    History of emerging infectious diseases
    The discovery of disease-causing pathogens is an important activity in the field of medical science, as many viruses, bacteria, protozoa, fungi, helminthes and prions are identified as a confirmed or potential pathogen...


The profession


To date, few universities offer epidemiology as a course of study at the undergraduate level. Many epidemiologists are physicians, or hold graduate degrees such as a Master of Public Health
Master of Public Health
The Master of Public Health and the Doctor of Public Health are multi-disciplinary professional degrees awarded for studies in areas related to public health....

 (MPH), Master of Science
Master of Science
A Master of Science is a postgraduate academic master's degree awarded by universities in many countries. The degree is typically studied for in the sciences including the social sciences.-Brazil, Argentina and Uruguay:...

 or Epidemiology (MSc.). Doctorate
Doctorate
A doctorate is an academic degree or professional degree that in most countries refers to a class of degrees which qualify the holder to teach in a specific field, A doctorate is an academic degree or professional degree that in most countries refers to a class of degrees which qualify the holder...

s include the Doctor of Public Health (DrPH), Doctor of Pharmacy
Doctor of Pharmacy
A Doctor of Pharmacy is a professional doctorate degree in pharmacy. In some countries, it is a first professional degree, and a prerequisite for licensing to exercise the profession of pharmacist.-Kenya :...

 (PharmD), Doctor of Philosophy
Doctor of Philosophy
Doctor of Philosophy, abbreviated as Ph.D., PhD, D.Phil., or DPhil , in English-speaking countries, is a postgraduate academic degree awarded by universities...

 (PhD), Doctor of Science
Doctor of Science
Doctor of Science , usually abbreviated Sc.D., D.Sc., S.D. or Dr.Sc., is an academic research degree awarded in a number of countries throughout the world. In some countries Doctor of Science is the name used for the standard doctorate in the sciences, elsewhere the Sc.D...

 (ScD), or for clinically trained physicians, Doctor of Medicine
Doctor of Medicine
Doctor of Medicine is a doctoral degree for physicians. The degree is granted by medical schools...

 (MD), Doctor of Osteopathic Medicine
Doctor of Osteopathic Medicine
Doctor of Osteopathic Medicine is a professional doctoral degree for physicians in the United States. Holders of the MD degree, Doctor of Medicine, have the same rights, privileges and responsibilities as osteopathic physicians in the United States.The American Osteopathic Association’s Commission...

 (DO), and Doctor of Veterinary Medicine (DVM). In the United Kingdom, the title of 'doctor' is by long custom used to refer to general medical practitioners, whose professional degrees are usually those of Bachelor of Medicine and Surgery
Bachelor of Medicine and Surgery
Bachelor of Medicine, Bachelor of Surgery, or in Latin Medicinae Baccalaureus, Baccalaureus Chirurgiae , are the two first professional degrees awarded upon graduation from medical school in medicine and surgery by universities in various countries...

 (MBBS or MBChB). As public health/health protection practitioners, epidemiologists work in a number of different settings. Some epidemiologists work 'in the field'; i.e., in the community, commonly in a public health/health protection service and are often at the forefront of investigating and combating disease outbreaks. Others work for non-profit organizations, universities, hospitals and larger government entities such as the Centers for Disease Control and Prevention
Centers for Disease Control and Prevention
The Centers for Disease Control and Prevention are a United States federal agency under the Department of Health and Human Services headquartered in Druid Hills, unincorporated DeKalb County, Georgia, in Greater Atlanta...

 (CDC), the Health Protection Agency
Health Protection Agency
The Health Protection Agency, or, in Welsh, Yr Asiantaeth Diogelu Iechyd is a statutory corporation. It is an independent UK organisation that was set up by the government in 2003 to protect the public from threats to their health from infectious diseases and environmental hazards...

, The World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 (WHO), or the Public Health Agency of Canada
Public Health Agency of Canada
The Public Health Agency of Canada is an agency of the Government of Canada that is responsible for public health, emergency preparedness, and response and infectious and chronic disease control and prevention...

. Epidemiologists can also work in for-profit organizations such as pharmaceutical and medical device companies in groups such as market research or clinical development.

The practice


Epidemiologists employ a range of study designs from the observational to experimental and generally categorized as descriptive, analytic (aiming to further examine known associations or hypothesized relationships), and experimental (a term often equated with clinical or community trials of treatments and other interventions). Epidemiological studies are aimed, where possible, at revealing unbiased relationships between exposures such as alcohol or smoking, biological agents, stress
Stress (medicine)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...

, or chemicals
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 to mortality
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

 or morbidity. The identification of causal relationships between these exposures and outcomes is an important aspect of epidemiology. Modern epidemiologists use informatics
Health informatics
.Health informatics is a discipline at the intersection of information science, computer science, and health care...

 as a tool.

The term 'epidemiologic triad' is used to describe the intersection of Host, Agent, and Environment in analyzing an outbreak.

As causal inference


Although epidemiology is sometimes viewed as a collection of statistical tools used to elucidate the associations of exposures to health outcomes, a deeper understanding of this science is that of discovering causal relationships.

It is nearly impossible to say with perfect accuracy how even the most simple physical systems behave beyond the immediate future, much less the complex field of epidemiology, which draws on biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, sociology
Sociology
Sociology is the study of society. It is a social science—a term with which it is sometimes synonymous—which uses various methods of empirical investigation and critical analysis to develop a body of knowledge about human social activity...

, mathematics
Mathematics
Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

, statistics
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

, anthropology
Anthropology
Anthropology is the study of humanity. It has origins in the humanities, the natural sciences, and the social sciences. The term "anthropology" is from the Greek anthrōpos , "man", understood to mean mankind or humanity, and -logia , "discourse" or "study", and was first used in 1501 by German...

, psychology
Psychology
Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

, and policy
Policy
A policy is typically described as a principle or rule to guide decisions and achieve rational outcome. The term is not normally used to denote what is actually done, this is normally referred to as either procedure or protocol...

; "Correlation does not imply causation
Correlation does not imply causation
"Correlation does not imply causation" is a phrase used in science and statistics to emphasize that correlation between two variables does not automatically imply that one causes the other "Correlation does not imply causation" (related to "ignoring a common cause" and questionable cause) is a...

" is a common theme for much of the epidemiological literature. For epidemiologists, the key is in the term inference
Inference
Inference is the act or process of deriving logical conclusions from premises known or assumed to be true. The conclusion drawn is also called an idiomatic. The laws of valid inference are studied in the field of logic.Human inference Inference is the act or process of deriving logical conclusions...

. Epidemiologists use gathered data and a broad range of biomedical and psychosocial theories in an iterative way to generate or expand theory, to test hypotheses, and to make educated, informed assertions about which relationships are causal, and about exactly how they are causal. Epidemiologists Rothman and Greenland emphasize that the "one cause - one effect" understanding is a simplistic mis-belief. Most outcomes, whether disease or death, are caused by a chain or web consisting of many component causes. Causes can be distinguished as necessary, sufficient or probabilistic conditions. If a necessary condition can be identified and controlled (e.g., antibodies to a disease agent), the harmful outcome can be avoided.

Bradford-Hill criteria


In 1965 Austin Bradford Hill
Austin Bradford Hill
Sir Austin Bradford Hill FRS , English epidemiologist and statistician, pioneered the randomized clinical trial and, together with Richard Doll, was the first to demonstrate the connection between cigarette smoking and lung cancer...

 detailed criteria for assessing evidence of causation. These guidelines are sometimes referred to as the Bradford-Hill criteria
Bradford-Hill criteria
The Bradford Hill criteria, otherwise known as Hill's criteria for causation, are a group of minimal conditions necessary to provide adequate evidence of a causal relationship between an incidence and a consequence, established by the English epidemiologist Sir Austin Bradford Hill in 1965.The...

, but this makes it seem like it is some sort of checklist. For example, Phillips and Goodman (2004) note that they are often taught or referenced as a checklist for assessing causality, despite this not being Hill's intention. Hill himself said "None of my nine viewpoints can bring indisputable evidence for or against the cause-and-effect hypothesis and none can be required sine qua non".
  1. Strength: A small association does not mean that there is not a causal effect, though the larger the association, the more likely that it is causal.
  2. Consistency: Consistent findings observed by different persons in different places with different samples strengthens the likelihood of an effect.
  3. Specificity: Causation is likely if a very specific population at a specific site and disease with no other likely explanation. The more specific an association between a factor and an effect is, the bigger the probability of a causal relationship.
  4. Temporality: The effect has to occur after the cause (and if there is an expected delay between the cause and expected effect, then the effect must occur after that delay).
  5. Biological gradient: Greater exposure should generally lead to greater incidence of the effect. However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an inverse proportion is observed: greater exposure leads to lower incidence.
  6. Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that knowledge of the mechanism is limited by current knowledge).
  7. Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of an effect. However, Hill noted that "... lack of such [laboratory] evidence cannot nullify the epidemiological effect on associations".
  8. Experiment: "Occasionally it is possible to appeal to experimental evidence".
  9. Analogy: The effect of similar factors may be considered.

Legal interpretation


Epidemiological studies can only go to prove that an agent could have caused, but not that it did cause, an effect in any particular case:

"Epidemiology is concerned with the incidence
Incidence (epidemiology)
Incidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.Incidence proportion is the...

 of disease in populations and does not address the question of the cause of an individual's disease. This question, sometimes referred to as specific causation, is beyond the domain of the science of epidemiology. Epidemiology has its limits at the point where an inference is made that the relationship between an agent and a disease is causal (general causation) and where the magnitude of excess risk attributed to the agent has been determined; that is, epidemiology addresses whether an agent can cause a disease, not whether an agent did cause a specific plaintiff's disease."


In United States law, epidemiology alone cannot prove that a causal association does not exist in general. Conversely, it can be (and is in some circumstances) taken by US courts, in an individual case, to justify an inference that a causal association does exist, based upon a balance of probability
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...

.

The subdiscipline of forensic epidemiology is directed at the investigation of specific causation of disease or injury in individuals or groups of individuals in instances in which causation is disputed or is unclear, for presentation in legal settings.

Advocacy


As a public health
Public health
Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals" . It is concerned with threats to health based on population health...

 discipline, epidemiologic evidence is often used to advocate
Advocacy
Advocacy is a political process by an individual or a large group which normally aims to influence public-policy and resource allocation decisions within political, economic, and social systems and institutions; it may be motivated from moral, ethical or faith principles or simply to protect an...

 both personal measures like diet change and corporate measures like removal of junk food
Junk food
Junk food is an informal term applied to some foods that are perceived to have little or no nutritional value ; to products with nutritional value, but which also have ingredients considered unhealthy when regularly eaten; or to those considered unhealthy to consume at all...

 advertising, with study findings disseminated to the general public to help people to make informed decisions about their health. Often the uncertainties about these findings are not communicated well; news articles often prominently report the latest result of one study with little mention of its limitations, caveats, or context. Epidemiological tools have proved effective in establishing major causes of diseases like cholera
Cholera
Cholera is an infection of the small intestine that is caused by the bacterium Vibrio cholerae. The main symptoms are profuse watery diarrhea and vomiting. Transmission occurs primarily by drinking or eating water or food that has been contaminated by the diarrhea of an infected person or the feces...

 and lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...

, but experience difficulty in regards to more subtle health issues where causation is not as clear. Notably, conclusions drawn from observational studies may be reconsidered as later data from randomized controlled trial
Randomized controlled trial
A randomized controlled trial is a type of scientific experiment - a form of clinical trial - most commonly used in testing the safety and efficacy or effectiveness of healthcare services or health technologies A randomized controlled trial (RCT) is a type of scientific experiment - a form of...

s becomes available, as was the case with the association between the use of hormone replacement therapy
Hormone replacement therapy (menopause)
Hormone replacement therapy is a system of medical treatment for surgically menopausal, perimenopausal and to a lesser extent postmenopausal women...

 and cardiac risk.

Population-based health management


Epidemiological practice and the results of epidemiological analysis make a significant contribution to emerging population-based health management frameworks.

Population-based health management encompasses the ability to:
  • Assess the health states and health needs of a target population;
  • Implement and evaluate interventions that are designed to improve the health of that population; and
  • Efficiently and effectively provide care for members of that population in a way that is consistent with the community's cultural, policy and health resource values.


Modern population-based health management is complex, requiring a multiple set of skills (medical, political, technological, mathematical etc.) of which epidemiological practice and analysis is a core component, that is unified with management science to provide efficient and effective health care and health guidance to a population. This task requires the forward looking ability of modern risk management approaches that transform health risk factors, incidence, prevalence and mortality statistics (derived from epidemiological analysis) into management metrics that not only guide how a health system responds to current population health issues, but also how a health system can be managed to better respond to future potential population health issues.

Examples of organizations that use population-based health management that leverage the work and results of epidemiological practice include Canadian Strategy for Cancer Control, Health Canada Tobacco Control Programs, Rick Hansen Foundation, Canadian Tobacco Control Research Initiative.

Each of these organizations use a population-based health management framework called Life at Risk that combines epidemiological quantitative analysis with demographics, health agency operational research and economics to perform:
  • Population Life Impacts Simulations: Measurement of the future potential impact of disease upon the population with respect to new disease cases, prevalence, premature death as well as potential years of life lost from disability and death;
  • Labour Force Life Impacts Simulations: Measurement of the future potential impact of disease upon the labour force with respect to new disease cases, prevalence, premature death and potential years of life lost from disability and death;
  • Economic Impacts of Disease Simulations: Measurement of the future potential impact of disease upon private sector disposable income impacts (wages, corporate profits, private health care costs) and public sector disposable income impacts (personal income tax, corporate income tax, consumption taxes, publicly funded health care costs).

Case series


Case-series may refer to the qualititative study of the experience of a single patient, or small group of patients with a similar diagnosis, or to a statistical technique comparing periods during which patients are exposed to some factor with the potential to produce illness with periods when they are unexposed.

The former type of study is purely descriptive and cannot be used to make inferences about the general population of patients with that disease. These types of studies, in which an astute clinician identifies an unusual feature of a disease or a patient's history, may lead to formulation of a new hypothesis. Using the data from the series, analytic studies could be done to investigate possible causal factors. These can include case control studies or prospective studies. A case control study would involve matching comparable controls without the disease to the cases in the series. A prospective study would involve following the case series over time to evaluate the disease's natural history.

The latter type, more formally described as self-controlled case-series studies, divide individual patient follow-up time into exposed and unexposed periods and use fixed-effects Poisson regression processes to compare the incidence rate of a given outcome between exposed and unexposed periods. This technique has been extensively used in the study of adverse reactions to vaccination, and has been shown in some circumstances to provide statistical power comparable to that available in cohort studies.

Case control studies


Case control studies select subjects based on their disease status. A group of individuals that are disease positive (the "case" group) is compared with a group of disease negative individuals (the "control" group). The control group should ideally come from the same population that gave rise to the cases. The case control study looks back through time at potential exposures that both groups (cases and controls) may have encountered. A 2x2 table is constructed, displaying exposed cases (A), exposed controls (B), unexposed cases (C) and unexposed controls (D). The statistic generated to measure association is the odds ratio
Odds ratio
The odds ratio is a measure of effect size, describing the strength of association or non-independence between two binary data values. It is used as a descriptive statistic, and plays an important role in logistic regression...

 (OR), which is the ratio of the odds of exposure in the cases (A/C) to the odds of exposure in the controls (B/D), i.e. OR = (A/C) / (B/D) .
..... Cases Controls
Exposed A B
Unexposed C D


If the OR is clearly greater than 1, then the conclusion is "those with the disease are more likely to have been exposed," whereas if it is close to 1 then the exposure and disease are not likely associated. If the OR is far less than one, then this suggests that the exposure is a protective factor in the causation of the disease.

Case control studies are usually faster and more cost effective than cohort studies, but are sensitive to bias (such as recall bias
Recall bias
In psychology, recall bias is a type of systematic bias which occurs when the way a survey respondent answers a question is affected not just by the correct answer, but also by the respondent's memory. This can affect the results of the survey. As a hypothetical example, suppose that a survey in...

 and selection bias
Selection bias
Selection bias is a statistical bias in which there is an error in choosing the individuals or groups to take part in a scientific study. It is sometimes referred to as the selection effect. The term "selection bias" most often refers to the distortion of a statistical analysis, resulting from the...

). The main challenge is to identify the appropriate control group; the distribution of exposure among the control group should be representative of the distribution in the population that gave rise to the cases. This can be achieved by drawing a random sample from the original population at risk. This has as a consequence that the control group can contain people with the disease under study when the disease has a high attack rate in a population.

Cohort studies


Cohort studies select subjects based on their exposure status. The study subjects should be at risk of the outcome under investigation at the beginning of the cohort study; this usually means that they should be disease free when the cohort study starts. The cohort is followed through time to assess their later outcome status. An example of a cohort study would be the investigation of a cohort of smokers and non-smokers over time to estimate the incidence of lung cancer. The same 2x2 table is constructed as with the case control study. However, the point estimate generated is the Relative Risk
Relative risk
In statistics and mathematical epidemiology, relative risk is the risk of an event relative to exposure. Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group....

 (RR), which is the probability of disease for a person in the exposed group, Pe = A / (A+B) over the probability of disease for a person in the unexposed group, Pu = C / (C+D), i.e. RR = Pe / Pu.
..... Case Non case Total
Exposed A B (A+B)
Unexposed C D (C+D)


As with the OR, a RR greater than 1 shows association, where the conclusion can be read "those with the exposure were more likely to develop disease."

Prospective studies have many benefits over case control studies. The RR is a more powerful effect measure than the OR, as the OR is just an estimation of the RR, since true incidence cannot be calculated in a case control study where subjects are selected based on disease status. Temporality can be established in a prospective study, and confounders are more easily controlled for. However, they are more costly, and there is a greater chance of losing subjects to follow-up based on the long time period over which the cohort is followed.

Outbreak investigation

For information on investigation of infectious disease
Infectious disease
Infectious diseases, also known as communicable diseases, contagious diseases or transmissible diseases comprise clinically evident illness resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism...

 outbreaks, please see outbreak investigation.

Validity: precision and bias


Different fields in epidemiology have different levels of validity. One way to assess the validity of findings is the ratio of false-positives (claimed effects that are not correct) to false-negatives (studies which fail to support a true effect). To take the field off genetic epidemiology, candidate-gene studies produced over 100 false-positive findings for each false-negative. By contrast genome-wide association appear close to the reverse, with only one false positive for every 100 or more false-negatives . This ratio has improved over time in genetic epidemiology as the field has adopted stringent criteria. By contrast other epidemiological fields have not required such rigorous reporting and are much less reliable as a result

Random error


Random error is the result of fluctuations around a true value because of sampling variability. Random error is just that: random. It can occur during data collection, coding, transfer, or analysis. Examples of random error include: poorly worded questions, a misunderstanding in interpreting an individual answer from a particular respondent, or a typographical error during coding. Random error affects measurement in a transient, inconsistent manner and it is impossible to correct for random error.

There is random error in all sampling procedures. This is called sampling error
Sampling error
-Random sampling:In statistics, sampling error or estimation error is the error caused by observing a sample instead of the whole population. The sampling error can be found by subtracting the value of a parameter from the value of a statistic...

.

Precision in epidemiological variables is a measure of random error. Precision is also inversely related to random error, so that to reduce random error is to increase precision. Confidence intervals are computed to demonstrate the precision of relative risk estimates. The narrower the confidence interval, the more precise the relative risk estimate.

There are two basic ways to reduce random error in an epidemiological study. The first is to increase the sample size of the study. In other words, add more subjects to your study. The second is to reduce the variability in measurement in the study. This might be accomplished by using a more precise measuring device or by increasing the number of measurements.

Note, that if sample size or number of measurements are increased, or a more precise measuring tool is purchased, the costs of the study are usually increased. There is usually an uneasy balance between the need for adequate precision and the practical issue of study cost.

Systematic error


A systematic error or bias occurs when there is a difference between the true value (in the population) and the observed value (in the study) from any cause other than sampling variability. An example of systematic error is if, unbeknown to you, the pulse oximeter
Pulse oximeter
A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times...

 you are using is set incorrectly and adds two points to the true value each time a measurement is taken. The measuring device could be precise but not accurate
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

. Because the error happens in every instance, it is systematic. Conclusions you draw based on that data will still be incorrect. But the error can be reproduced in the future (e.g., by using the same mis-set instrument).

A mistake in coding that affects all responses for that particular question is another example of a systematic error.

The validity of a study is dependent on the degree of systematic error. Validity is usually separated into two components:
  • Internal validity
    Internal validity
    Internal validity is the validity of inferences in scientific studies, usually based on experiments as experimental validity.- Details :...

     is dependent on the amount of error in measurements, including exposure, disease, and the associations between these variables. Good internal validity implies a lack of error in measurement and suggests that inferences may be drawn at least as they pertain to the subjects under study.

  • External validity
    External validity
    External validity is the validity of generalized inferences in scientific studies, usually based on experiments as experimental validity....

     pertains to the process of generalizing the findings of the study to the population from which the sample was drawn (or even beyond that population to a more universal statement). This requires an understanding of which conditions are relevant (or irrelevant) to the generalization. Internal validity is clearly a prerequisite for external validity.

Selection bias

Selection bias
Selection bias
Selection bias is a statistical bias in which there is an error in choosing the individuals or groups to take part in a scientific study. It is sometimes referred to as the selection effect. The term "selection bias" most often refers to the distortion of a statistical analysis, resulting from the...

 is one of three types of bias that can threaten the validity of a study. Selection bias occurs when study subjects are selected or become part of the study as a result of a third, unmeasured variable which is associated with both the exposure and outcome of interest.

Examples of selection bias are volunteer bias (the opposite of which is non-response bias) in which participants and non participants differ in terms of exposure and outcome. For instance, it has repeatedly been noted that cigarette smokers and non smokers tend to differ in their study participation rates. (Sackett D cites the example of Seltzer et al., in which 85% of non smokers and 67% of smokers returned mailed questionnaires) It is important to note that such a difference in response will not lead to bias if it is not also associated with a systematic difference in outcome between the two response groups.
Confounding

Confounding
Confounding
In statistics, a confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable...

 has traditionally been defined as bias arising from the co-occurrence or mixing of effects of extraneous factors, referred to as confounders, with the main effect(s) of interest. A more recent definition of confounding invokes the notion of counterfactual effects. According to this view, when one observes an outcome of interest, say Y=1 (as opposed to Y=0), in a given population A which is entirely exposed (i.e. exposure X=1 for every unit of the population) the risk of this event will be RA1. The counterfactual or unobserved risk RA0 corresponds to the risk which would have been observed if these same individuals had been unexposed (i.e. X=0 for every unit of the population). The true effect of exposure therefore is: RA1 - RA0 (if one is interested in risk differences) or RA1/RA0 (if one is interested in relative risk). Since the counterfactual risk RA0 is unobservable we approximate it using a second population B and we actually measure the following relations: RA1 - RB0 or RA1/RB0. In this situation, confounding occurs when RA0 ≠ RB0.

(NB: Example assumes binary outcome and exposure variables.)
Information bias

Information bias
Information bias (epidemiology)
-Definition:Also referred to as observational bias and misclassification. A Dictionary of Epidemiology, sponsored by the International Epidemiological Association, defines this as the following:...

 is bias arising from systematic error in the assessment of a variable. An example of this is recall bias. A typical example is again provided by Sackett in his discussion of a study examining the effect of specific exposures on fetal health: "in questioning mothers whose recent pregnancies had ended in fetal death or malformation (cases) and a matched group of mothers whose pregnancies ended normally (controls) it was found that 28%; of the former, but only 20%,; of the latter, reported exposure to drugs which could not be substantiated either in earlier prospective
interviews or in other health records". In this example, recall bias probably occurred as a result of women who had had miscarriages having an apparent tendency to better recall and therefore report previous exposures.

Journals


A list of journals:
General journals:
  • American Journal of Epidemiology
    American Journal of Epidemiology
    The American Journal of Epidemiology is a peer reviewed journal for empirical research findings, opinion pieces and methodological developments in the field of epidemiological research. It is published semimonthly, and has an impact of 5.454, ranking it 6th out of 105 Journals in the field....

  • Canadian Journal of Epidemiology and Biostatistics
  • Epidemiologic Reviews
  • Epidemiology
    Epidemiology (journal)
    EPIDEMIOLOGY is a bi-monthly, peer-reviewed journal for epidemiologic research, published by Lippincott Williams & Wilkins.The journal publishes original research from all fields of epidemiology, as well as review articles, meta-analyses, novel hypotheses, descriptions and applications of new...

  • International Journal of Epidemiology
    International Journal of Epidemiology
    The International Journal of Epidemiology is a bimonthly peer-reviewed medical journal covering research in epidemiology. It is the official journal of the International Epidemiological Association. The journal is a member of the Committee on Publication Ethics....

  • Annals of Epidemiology
    Annals of Epidemiology
    The Annals of Epidemiology is a peer reviewed journal devoted to epidemiological research and is published as the official journal for American College of Epidemiology....

  • Journal of Epidemiology and Community Health
  • European Journal of Epidemiology
    European Journal of Epidemiology
    The European Journal of Epidemiology is a peer-reviewed medical journal on the epidemiology of communicable and non-communicable diseases and their control. The editor in chief is Albert Hofman...

  • Emerging themes in epidemiology
    Emerging themes in epidemiology
    Emerging Themes in Epidemiology is an online open access peer-reviewed medical journal. It is managed by current doctoral students and recent PhD graduates in the United Kingdom and Canada...

  • Epidemiologic Perspectives and Innovations
  • Eurosurveillance


Specialty journals:

Areas


By physiology/disease:
  • Infectious disease epidemiology
  • Occupational Injury & Illness epidemiology
  • Cardiovascular disease epidemiology
  • Cancer
    Cancer
    Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

     epidemiology
  • Neuroepidemiology
    Neuroepidemiology
    Neuroepidemiology is a branch of epidemiology involving the study of neurological disease distribution and determinants of frequency in human populations. The term was first introduced by Dr. Len Kurland, Dr. Milton Alter, and Dr. John Kurtzke in 1967...

  • Epidemiology of Aging
  • Oral/Dental epidemiology
  • Reproductive epidemiology
  • Obesity
    Obesity
    Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

    /diabetes epidemiology
  • Renal epidemiology
  • Intestinal epidemiology
  • Psychiatric epidemiology
  • Veterinary epidemiology
  • Epidemiology of zoonosis
    Zoonosis
    A zoonosis or zoonoseis any infectious disease that can be transmitted from non-human animals to humans or from humans to non-human animals . In a study of 1415 pathogens known to affect humans, 61% were zoonotic...

  • Respiratory Epidemiology
  • Pediatric Epidemiology
  • Quantitative parasitology
    Quantitative parasitology
    -Counting parasites:Quantifying parasites in a sample of hosts or comparing measures of infection across two or more samples can be challenging.The parasitic infection of a sample of hosts inherently exhibits a complex pattern that cannot be adequately quantified by a single statistical measure...



By methodological approach:
  • Environmental epidemiology
    Environmental epidemiology
    Environmental epidemiology is the branch of epidemiology concerned with discovery of the environmental exposures that contribute to or protect against injuries, illnesses, developmental conditions, disabilities, and deaths; and identification of public health and health care actions to avoid,...

  • Economic epidemiology
    Economic epidemiology
    Economic epidemiology is a field at the intersection of epidemiology and economics. Its premise is to incorporate incentives for healthy behavior and their attendant behavioral responses into an epidemiological context to better understand how diseases are transmitted...

  • Clinical epidemiology
  • Conflict epidemiology
    Conflict epidemiology
    The emerging field of conflict epidemiology offers a more accurate method to measure deaths caused during violent conflicts or wars that can generate more reliable numbers than before to guide decision-makers....

  • Cognitive epidemiology
    Cognitive epidemiology
    Cognitive epidemiology is a field of research that examines the associations between intelligence test scores and health, more specifically morbidity and mortality. Typically, test scores are obtained at an early age, and compared to later morbidity and mortality...

  • Genetic epidemiology
    Genetic epidemiology
    Genetic epidemiology is the study of the role of genetic factors in determining health and disease in families and in populations, and the interplay of such genetic factors with environmental factors...

  • Molecular epidemiology
    Molecular epidemiology
    Molecular epidemiology is a branch of medical science that focuses on the contribution of potential genetic and environmental risk factors, identified at the molecular level, to the etiology, distribution and prevention of disease within families and across populations. This field has emerged from...

  • Nutritional epidemiology
    Nutritional epidemiology
    Nutritional epidemiology is a relatively new field of medical research that studies the relationship between nutrition and health. Diet and physical activity are difficult to measure accurately, which may partly explain why nutrition has received less attention than other risk factors for disease...

  • Social epidemiology
    Social epidemiology
    Social epidemiology is defined as "The branch of epidemiology that studies the social distribution and social determinants of health," that is, "both specific features of, and pathways by which, societal conditions affect health."...

  • Lifecourse epidemiology
  • Epi methods development / Biostatistics
    Biostatistics
    Biostatistics is the application of statistics to a wide range of topics in biology...

  • Meta-analysis
    Meta-analysis
    In statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...

  • Spatial epidemiology
    Spatial epidemiology
    Spatial epidemiology is a subfield of health geography focused on the study of the spatial distribution of disease.-See also:General topics* Cluster * Complete spatial randomness* Geographic information system* Geographic information science...

  • Tele-epidemiology
    Tele-epidemiology
    Tele-epidemiology is a methodological and application area of epidemiology concerned with the application of space-based systems in the study of the space and time distribution of health events or disease process in populations.In this broader sense, the term includes...

  • Biomarker epidemiology
  • Pharmacoepidemiology
    Pharmacoepidemiology
    Pharmacoepidemiology is the study of the use of and the effects of drugs in large numbers of people.To accomplish this study, pharmacoepidemiology borrows from both pharmacology and epidemiology. Thus, pharmacoepidemiology is the bridge between both pharmacology and epidemiology...

  • Primary care epidemiology
  • Infection control and hospital epidemiology
  • Public Health practice epidemiology
  • Surveillance
    Surveillance
    Surveillance is the monitoring of the behavior, activities, or other changing information, usually of people. It is sometimes done in a surreptitious manner...

     epidemiology (Clinical surveillance
    Clinical surveillance
    Clinical surveillance refers to the surveillance of health data about a clinical syndrome that has a significant impact on public health, which is then used to drive decisions about health policy and health education...

    )
  • Disease Informatics
    Disease Informatics
    Disease Informatics is the application of Information Science in defining the diseases with least error, identifying most of the targets to combat a cluster of diseases and designing a holistic solution to the problem....



External links