The science of epidemiology has matured significantly from the times of 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...

 and 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...

. The techniques for gathering and analyzing epidemiological data vary depending on the type of disease being monitored but each study will have overarching similarities.

Outline of the process of an epidemiological study

1. Establish that a problem exists
   • Full epidemiological studies are expensive and laborious undertakings. Before any study is started, a case must be made for the importance of the research.
2. Confirm the homogeneity of the events
   • Any conclusions drawn from inhomogeneous cases will be suspicious. All events or occurrences of the disease must be true cases of the disease.
3. Collect all the events
   • It is important to collect as much information as possible about each event in order to inspect a large number of possible risk factors. The events may be collected from varied methods of epidemiological study or from censuses or hospital records.
   • The events can be characterized by 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...
     
      rates and prevalence
     Prevalence
     In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
     
      rates.
4. Characterize the events as to epidemiological factors
   1. Predisposing factors
      • Non-environmental factors that increase the likelihood of getting a disease. Genetic history, age, and gender are examples.
   2. Enabling/disabling factors
      • Factors relating to the environment that either increase or decrease the likelihood of disease. Exercise and good diet are examples of disabling factors. A weakened immune system and poor nutrition are examples of enabling factors.
   3. Precipitation factors
      • This factor is the most important in that it identifies the source of exposure. It may be a germ, toxin or gene.
   4. Reinforcing factors
      • These are factors that compound the likelihood of getting a disease. They may include repeated exposure or excessive environmental stresses.
5. Look for patterns and trends
   • Here one looks for similarities in the cases which may identify major risk factors for contracting the disease. Epidemic curves may be used to identify such risk factors.
6. Formulate a hypothesis
   • If a trend has been observed in the cases, the researcher may postulate as to the nature of the relationship between the potential disease-causing agent and the disease.
7. Test the hypothesis
   • Because epidemiological studies can rarely be conducted in a laboratory the results are often polluted by uncontrollable variations in the cases. This often makes the results difficult to interpret. Two methods have evolved to assess the strength of the relationship between the disease causing agent and the disease.
   • Koch's postulates
     Koch's postulates
     Koch's postulates are four criteria designed to establish a causal relationship between a causative microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884 and refined and published by Koch in 1890...
     
      were the first criteria developed for epidemiological relationships. Because they only work well for highly contagious bacteria and toxins, this method is largely out of favor.
   • Bradford-Hill Criteria are the current standards for epidemiological relationships. A relationship may fill all, some, or none of the criteria and still be true.
8. Publish the results

Measures

Epidemiologist are famous for their use of rates. Each measure serves to characterize the disease giving valuable information about contagiousness, incubation period, duration, and mortality of the disease.

Measures of occurrence

1. 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...
   
    measures
   1. Incidence rate, where cases included are defined using a case definition
      Case definition
      A case definition from a broader sense encompassing its use in more than just medical terms. A case is a request for a service for a subject - customer, employee, or citizen. The case has supporting information - documents, data, forms, etc. - as part of the request for service...
      
      
   2. Hazard rate
   3. Cumulative incidence
      Cumulative incidence
      Cumulative incidence or incidence proportion is a measure of frequency, as in epidemiology, where it is a measure of disease frequency during a period of time...
      
      
2. Prevalence
   Prevalence
   In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
   
    measures
   1. Point prevalence
      Point prevalence
      In epidemiology, point prevalence is a measure of the proportion of people in a population who have a disease or condition at a particular time, such as a particular date. It is like a snap shot of the disease in time. It can be used for statistics on the occurrence of chronic diseases...
      
      
   2. Period prevalence
      Period prevalence
      In epidemiology, Period prevalence is the proportion of the population with a given disease or condition over a specific period of time. It could describe how many people in a population had a cold over the cold season in 2006, for example....
      
      

Measures of association

1. Relative measures
   1. Risk ratio
   2. Rate ratio
      Rate ratio
      A rate ratio in epidemiology is calculated to compare the rate of events occurring at any given point in time.Rate Ratio = Incidence Rate 1/Incidence Rate 2Where incidence rate is the occurrence of an event over person-time, for example person-years....
      
      
   3. 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...
      
      
   4. Hazard ratio
      Hazard ratio
      In survival analysis, the hazard ratio is the ratio of the hazard rates corresponding to the conditions described by two sets of explanatory variables. For example, in a drug study, the treated population may die at twice the rate per unit time as the control population. The hazard ratio would be...
      
      
2. Absolute measures
   1. Absolute risk reduction
      Absolute risk reduction
      In epidemiology, the absolute risk reduction or risk difference is the decrease in risk of a given activity or treatment in relation to a control activity or treatment. It is the inverse of the number needed to treat....
      
      
   2. Attributable risk
      Attributable risk
      In epidemiology, attributable risk is the difference in rate of a condition between an exposed population and an unexposed population.. Attributable risk is mostly calculated in cohort studies, where individuals are assembled on exposure status and followed over a period of time. Investigators...
      
      
      1. Attributable risk in exposed
      2. Percent attributable risk
      3. Levin’s attributable risk

Other measures

1. Virulence
   Virulence
   Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
   
    and Infectivity
   Infectivity
   In epidemiology, infectivity refers to the ability of a pathogen to establish an infection. More specifically, infectivity is a pathogen's capacity for horizontal transmission that is, how frequently it spreads among hosts that are not in a parent-child relationship...
   
   
2. Mortality rate
   Mortality rate
   Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...
   
    and Morbidity rate
3. Case fatality
   Case fatality
   In epidemiology, case fatality or fatality rate, is the ratio of deaths within a designated population of people with a particular condition, over a certain period of time. An example of a fatality rate would be 9 deaths per 10,000 people at risk per year...
   
   
4. Sensitivity (tests) and Specificity (tests)

External links

• Epidemiologic.org Epidemiologic Inquiry online weblog for epidemiology researchers
• Epidemiology Forum A discussion and forum community for epi analysis support and fostering questions, debates, and collaborations in epidemiology
• The Centre for Evidence Based Medicine at Oxford maintains an on-line "Toolbox" of evidence based medicine methods.
• Epimonitor has a comprehensive list of links to associations, agencies, bulletins, etc.
• Epidemiology for the Uninitiated On line text, with easy explanations.
• North Carolina Center for Public Health Preparedness Training On line training classes for epidemiology and related topics.
• People's Epidemiology Library