Double-blind
Encyclopedia
A blind or blinded experiment is a scientific experiment where some of the people involved are prevented from knowing certain information that might lead to conscious or subconscious
bias on their part, invalidating the results.
For example, when asking consumers to compare the tastes of different brands of a product, the identities of the product should be concealed – otherwise consumers will generally tend to prefer the brand they are familiar with. Similarly, when evaluating the effectiveness of a medical drug
, both the patients and the doctors who administer the drug may be kept in the dark about the dosage being applied in each case – to forestall any chance of a placebo effect
, observer bias, or conscious deception.
Blinding can be imposed on researchers, technicians, subjects, funders, or any combination of them. The opposite of a blind trial is an open trial
. Blind experiments are an important tool of the scientific method
, in many fields of research – from medicine
, forensics
, psychology
and the social sciences
, to natural sciences such as physics
and biology
and to market research
. In some disciplines, such as drug testing, blind experiments are considered essential. In other disciplines, blind experiments would be very useful, but they are totally impractical or unethical. An oft-cited example is in the field of developmental psychology
. Although it would be scientifically expedient to raise children under arbitrary experimental conditions, such as on a remote island with a fabricated enculturation, it is obviously a violation of ethics
and human rights.
The terms blind (adjective
) or to blind (transitive verb
) when used in this sense are figurative extensions of the literal idea of blindfold
ing someone. The terms masked or to mask may be used for the same concept. (This is commonly the case in ophthalmology
, where the word 'blind' is often used in the literal sense.)
, which set up a commission to investigate the claims of animal magnetism
proposed by Franz Mesmer
. Headed by Benjamin Franklin
and Antoine Lavoisier
, the commission carried out experiments asking mesmerists to identify objects that had previously been filled with "vital fluid", including trees and flasks of water. The subjects were unable to do so. The commission went on to examine claims involving the curing of "mesmerized" patients. They found that these patients did actually show signs of improved health, although the commission attributed this to the fact that these patients believed they would get better -- the first scientific suggestion of the now well-known placebo effect
.
Another early blind experiment was performed in 1799 by the British chemist Humphry Davy
. In studying the effects of nitrous oxide
(laughing gas) on human physiology, he deliberately did not tell his subjects what concentration of the gas they were breathing, or whether they were breathing ordinary air.
Blind experiments went on to be used outside of purely scientific settings. In 1817, a committee of scientists and musicians compared a Stradivarius
violin to one with a guitar-like design made by the naval engineer François Chanot. Both were played by a well-known violinist while the committee listened in the next room to avoid prejudice.
One of the first essays advocating a blinded approach to experiments in general came from Claude Bernard
in the latter half of the 19th century, who recommended that any scientific experiment be split between the theorist who conceives the experiment and a naive (and preferably uneducated) observer who registers the results without foreknowledge of the theory or hypothesis being tested. This suggestion contrasted starkly with the prevalent Enlightenment
-era attitude that scientific observation can only be objectively valid when undertaken by a well-educated, informed scientist.
s where information that could introduce bias or otherwise skew the result is withheld from the participants, but the experimenter will be in full possession of the facts.
In a single-blind experiment, the individual subjects do not know whether they are so-called "test" subjects or members of an "experimental control
" group. Single-blind experimental design is used where the experimenters either must know the full facts (for example, when comparing sham to real surgery) and so the experimenters cannot themselves be blind, or where the experimenters will not introduce further bias and so the experimenters need not be blind. However, there is a risk that subjects are influenced by interaction with the researchers – known as the experimenter's bias
. Single-blind trials are especially risky in psychology and social science research, where the experimenter has an expectation of what the outcome should be, and may consciously or subconsciously influence the behavior of the subject.
A classic example of a single-blind test is the "Pepsi challenge
". A marketing person prepares several cups of cola labeled "A" and "B". One set of cups has Pepsi, the others have Coca-Cola. The marketing person knows which soda is in which cup but is not supposed to reveal that information to the subjects. Volunteer subjects are encouraged to try the two cups of soda and polled for which ones they prefer. The problem with a single-blind test like this is the marketing person can give (unintentional or not) subconscious cues which bias the volunteer. In addition it is possible the marketing person could prepare the separate sodas differently (more ice in one cup, push one cup in front of the volunteer, etc.) which can cause a bias. If the marketing person is employed by the company which is producing the challenge there's always the possibility of a conflict of interests where the marketing person is aware that future income will be based on the results of the test.
, usually on human subjects, in an attempt to eliminate subjective bias on the part of both experimental subjects and the experimenters. In most cases, double-blind experiments are held to achieve a higher standard of scientific rigor.
In a double-blind experiment, neither the individuals nor the researchers know who belongs to the control group and the experimental group. Only after all the data have been recorded (and in some cases, analysed) do the researchers learn which individuals are which. Performing an experiment in double-blind fashion is a way to lessen the influence of the prejudices and unintentional physical cues on the results (the placebo effect
, observer bias, and experimenter's bias
). Random assignment
of the subject to the experimental or control group is a critical part of double-blind research design. The key that identifies the subjects and which group they belonged to is kept by a third party and not given to the researchers until the study is over.
Double-blind methods can be applied to any experimental situation where there is the possibility that the results will be affected by conscious or unconscious bias
on the part of the experimenter.
Computer-controlled experiments are sometimes also erroneously referred to as double-blind experiments, since software may not cause the type of direct bias between researcher and subject. Development of surveys presented to subjects through computers shows that bias can easily be built into the process. Voting systems are also examples where bias can easily be constructed into an apparently simple machine based system. In analogy to the human researcher described above, the part of the software that provides interaction with the human is presented to the subject as the blinded researcher, while the part of the software that defines the key is the third party. An example is the ABX test
, where the human subject has to identify an unknown stimulus X as being either A or B.
or an established drug) to have identical appearance (color, taste, etc.). Patients are randomly assigned to the control or experimental group and given random numbers by a study coordinator, who also encodes the drugs with matching random numbers. Neither the patients nor the researchers monitoring the outcome know which patient is receiving which treatment, until the study is over and the random code is broken.
Effective blinding can be difficult to achieve where the treatment is notably effective (indeed, studies have been suspended in cases where the tested drug combinations were so effective that it was deemed unethical to continue withholding the findings from the control group, and the general population), or where the treatment is very distinctive in taste or has unusual side-effects that allow the researcher and/or the subject to guess which group they were assigned to. It is also difficult to use the double blind method to compare surgical and non-surgical interventions (although sham surgery
, involving a simple incision, might be ethically permitted). A good clinical protocol will foresee these potential problems to ensure blinding is as effective as possible. It has also been argued that even in a double-blind experiment, general attitudes of the experimenter such as skepticism or enthusiasm towards the tested procedure can be subconsciously transferred to the test subjects.
Evidence-based medicine
practitioners prefer blinded randomised controlled trials
(RCTs), where that is a possible experimental design. These are high on the hierarchy of evidence; only a meta analysis of several well designed RCTs is considered more reliable.
and particle physics
experiments often involve large numbers of data analysts working together to extract quantitative data from complex datasets. In particular, the analysts want to report accurate systematic error
estimates for all of their measurements; this is difficult or impossible if one of the errors is observer bias. To remove this bias, the experimenters devise blind analysis techniques, where the experimental result is hidden from the analysts until they've agreed—based on properties of the data set other than the final value—that the analysis techniques are fixed.
One example of a blind analysis occurs in neutrino
experiments, like the Sudbury Neutrino Observatory
, where the experimenters wish to report the total number N of neutrinos seen. The experimenters have preexisting expectations about what this number should be, and these expectations must not be allowed to bias the analysis. Therefore, the experimenters are allowed to see an unknown fraction f of the dataset. They use these data to understand the backgrounds, signal-detection efficiencies, detector resolutions, etc.. However, since no one knows the "blinding fraction" f, no one has preexisting expectations about the meaningless neutrino count N' = N x f in the visible data; therefore, the analysis does not introduce any bias into the final number N which is reported. Another blinding scheme is used in B meson
analyses in experiments like BaBar
and CDF
; here, the crucial experimental parameter is a correlation between certain particle energies and decay times—which require an extremely complex and painstaking analysis—and particle charge signs, which are fairly trivial to measure. Analysts are allowed to work with all of the energy and decay data, but are forbidden from seeing the sign of the charge, and thus are unable to see the correlation (if any). At the end of the experiment, the correct charge signs are revealed; the analysis software is run once (with no subjective human intervention), and the resulting numbers are published. Searches for rare events, like electron neutrinos in MiniBooNE
or proton decay in Super-Kamiokande
, require a different class of blinding schemes.
The "hidden" part of the experiment—the fraction f for SNO, the charge-sign database for CDF—is usually called the "blindness box". At the end of the analysis period, one is allowed to "unblind the data" and "open the box".
Subconscious
The term subconscious is used in many different contexts and has no single or precise definition. This greatly limits its significance as a definition-bearing concept, and in consequence the word tends to be avoided in academic and scientific settings....
bias on their part, invalidating the results.
For example, when asking consumers to compare the tastes of different brands of a product, the identities of the product should be concealed – otherwise consumers will generally tend to prefer the brand they are familiar with. Similarly, when evaluating the effectiveness of a medical drug
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...
, both the patients and the doctors who administer the drug may be kept in the dark about the dosage being applied in each case – to forestall any chance of a placebo effect
Placebo effect
Placebo effect may refer to:* Placebo effect, the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work...
, observer bias, or conscious deception.
Blinding can be imposed on researchers, technicians, subjects, funders, or any combination of them. The opposite of a blind trial is an open trial
Open-label trial
An open-label trial or open trial is a type of clinical trial in which both the researchers and participants know which treatment is being administered....
. Blind experiments are an important tool of the scientific method
Scientific method
Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...
, in many fields of research – from medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, forensics
Forensics
Forensic science is the application of a broad spectrum of sciences to answer questions of interest to a legal system. This may be in relation to a crime or a civil action...
, 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 the social sciences
Social sciences
Social science is the field of study concerned with society. "Social science" is commonly used as an umbrella term to refer to a plurality of fields outside of the natural sciences usually exclusive of the administrative or managerial sciences...
, to natural sciences such as physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
and 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...
and to market research
Market research
Market research is any organized effort to gather information about markets or customers. It is a very important component of business strategy...
. In some disciplines, such as drug testing, blind experiments are considered essential. In other disciplines, blind experiments would be very useful, but they are totally impractical or unethical. An oft-cited example is in the field of developmental psychology
Developmental psychology
Developmental psychology, also known as human development, is the scientific study of systematic psychological changes, emotional changes, and perception changes that occur in human beings over the course of their life span. Originally concerned with infants and children, the field has expanded to...
. Although it would be scientifically expedient to raise children under arbitrary experimental conditions, such as on a remote island with a fabricated enculturation, it is obviously a violation of ethics
Ethics
Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...
and human rights.
The terms blind (adjective
Adjective
In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....
) or to blind (transitive verb
Transitive verb
In syntax, a transitive verb is a verb that requires both a direct subject and one or more objects. The term is used to contrast intransitive verbs, which do not have objects.-Examples:Some examples of sentences with transitive verbs:...
) when used in this sense are figurative extensions of the literal idea of blindfold
Blindfold
A blindfold is a garment, usually of cloth, tied to one's head to cover the eyes to disable the wearer's sight. It can be worn when the eyes are in a closed state and thus prevents the wearer from opening them...
ing someone. The terms masked or to mask may be used for the same concept. (This is commonly the case in ophthalmology
Ophthalmology
Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...
, where the word 'blind' is often used in the literal sense.)
History
The first recorded blind experiments were performed in 1784 by the French Academy of SciencesFrench Academy of Sciences
The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research...
, which set up a commission to investigate the claims of animal magnetism
Animal magnetism
Animal magnetism , in modern usage, refers to a person's sexual attractiveness or raw charisma. As postulated by Franz Mesmer in the 18th century, the term referred to a supposed magnetic fluid or ethereal medium believed to reside in the bodies of animate beings...
proposed by Franz Mesmer
Franz Mesmer
Franz Anton Mesmer , sometimes, albeit incorrectly, referred to as Friedrich Anton Mesmer, was a German physician with an interest in astronomy, who theorised that there was a natural energetic transference that occurred between all animated and inanimate objects that he called magnétisme animal ...
. Headed by Benjamin Franklin
Benjamin Franklin
Dr. Benjamin Franklin was one of the Founding Fathers of the United States. A noted polymath, Franklin was a leading author, printer, political theorist, politician, postmaster, scientist, musician, inventor, satirist, civic activist, statesman, and diplomat...
and Antoine Lavoisier
Antoine Lavoisier
Antoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...
, the commission carried out experiments asking mesmerists to identify objects that had previously been filled with "vital fluid", including trees and flasks of water. The subjects were unable to do so. The commission went on to examine claims involving the curing of "mesmerized" patients. They found that these patients did actually show signs of improved health, although the commission attributed this to the fact that these patients believed they would get better -- the first scientific suggestion of the now well-known placebo effect
Placebo effect
Placebo effect may refer to:* Placebo effect, the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work...
.
Another early blind experiment was performed in 1799 by the British chemist Humphry Davy
Humphry Davy
Sir Humphry Davy, 1st Baronet FRS MRIA was a British chemist and inventor. He is probably best remembered today for his discoveries of several alkali and alkaline earth metals, as well as contributions to the discoveries of the elemental nature of chlorine and iodine...
. In studying the effects of nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
(laughing gas) on human physiology, he deliberately did not tell his subjects what concentration of the gas they were breathing, or whether they were breathing ordinary air.
Blind experiments went on to be used outside of purely scientific settings. In 1817, a committee of scientists and musicians compared a Stradivarius
Stradivarius
The name Stradivarius is associated with violins built by members of the Stradivari family, particularly Antonio Stradivari. According to their reputation, the quality of their sound has defied attempts to explain or reproduce, though this belief is controversial...
violin to one with a guitar-like design made by the naval engineer François Chanot. Both were played by a well-known violinist while the committee listened in the next room to avoid prejudice.
One of the first essays advocating a blinded approach to experiments in general came from Claude Bernard
Claude Bernard
Claude Bernard was a French physiologist. He was the first to define the term milieu intérieur . Historian of science I. Bernard Cohen of Harvard University called Bernard "one of the greatest of all men of science"...
in the latter half of the 19th century, who recommended that any scientific experiment be split between the theorist who conceives the experiment and a naive (and preferably uneducated) observer who registers the results without foreknowledge of the theory or hypothesis being tested. This suggestion contrasted starkly with the prevalent Enlightenment
Age of Enlightenment
The Age of Enlightenment was an elite cultural movement of intellectuals in 18th century Europe that sought to mobilize the power of reason in order to reform society and advance knowledge. It promoted intellectual interchange and opposed intolerance and abuses in church and state...
-era attitude that scientific observation can only be objectively valid when undertaken by a well-educated, informed scientist.
Single-blind trials
Single-blind describes experimentExperiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
s where information that could introduce bias or otherwise skew the result is withheld from the participants, but the experimenter will be in full possession of the facts.
In a single-blind experiment, the individual subjects do not know whether they are so-called "test" subjects or members of an "experimental control
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...
" group. Single-blind experimental design is used where the experimenters either must know the full facts (for example, when comparing sham to real surgery) and so the experimenters cannot themselves be blind, or where the experimenters will not introduce further bias and so the experimenters need not be blind. However, there is a risk that subjects are influenced by interaction with the researchers – known as the experimenter's bias
Experimenter's bias
In experimental science, experimenter's bias is subjective bias towards a result expected by the human experimenter. David Sackett, in a useful review of biases in clinical studies, states that biases can occur in any one of seven stages of research:...
. Single-blind trials are especially risky in psychology and social science research, where the experimenter has an expectation of what the outcome should be, and may consciously or subconsciously influence the behavior of the subject.
A classic example of a single-blind test is the "Pepsi challenge
Pepsi Challenge
The Pepsi Challenge has been an ongoing marketing promotion run by PepsiCo since 1975. It is also the name of a cross country ski race at Giant's Ridge Ski Area in Biwabik, Minnesota, an event sponsored by Pepsi.-Method:...
". A marketing person prepares several cups of cola labeled "A" and "B". One set of cups has Pepsi, the others have Coca-Cola. The marketing person knows which soda is in which cup but is not supposed to reveal that information to the subjects. Volunteer subjects are encouraged to try the two cups of soda and polled for which ones they prefer. The problem with a single-blind test like this is the marketing person can give (unintentional or not) subconscious cues which bias the volunteer. In addition it is possible the marketing person could prepare the separate sodas differently (more ice in one cup, push one cup in front of the volunteer, etc.) which can cause a bias. If the marketing person is employed by the company which is producing the challenge there's always the possibility of a conflict of interests where the marketing person is aware that future income will be based on the results of the test.
Double-blind trials
Double-blind describes an especially stringent way of conducting an experimentExperiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
, usually on human subjects, in an attempt to eliminate subjective bias on the part of both experimental subjects and the experimenters. In most cases, double-blind experiments are held to achieve a higher standard of scientific rigor.
In a double-blind experiment, neither the individuals nor the researchers know who belongs to the control group and the experimental group. Only after all the data have been recorded (and in some cases, analysed) do the researchers learn which individuals are which. Performing an experiment in double-blind fashion is a way to lessen the influence of the prejudices and unintentional physical cues on the results (the placebo effect
Placebo effect
Placebo effect may refer to:* Placebo effect, the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work...
, observer bias, and experimenter's bias
Experimenter's bias
In experimental science, experimenter's bias is subjective bias towards a result expected by the human experimenter. David Sackett, in a useful review of biases in clinical studies, states that biases can occur in any one of seven stages of research:...
). Random assignment
Random sample
In statistics, a sample is a subject chosen from a population for investigation; a random sample is one chosen by a method involving an unpredictable component...
of the subject to the experimental or control group is a critical part of double-blind research design. The key that identifies the subjects and which group they belonged to is kept by a third party and not given to the researchers until the study is over.
Double-blind methods can be applied to any experimental situation where there is the possibility that the results will be affected by conscious or unconscious bias
Bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
on the part of the experimenter.
Computer-controlled experiments are sometimes also erroneously referred to as double-blind experiments, since software may not cause the type of direct bias between researcher and subject. Development of surveys presented to subjects through computers shows that bias can easily be built into the process. Voting systems are also examples where bias can easily be constructed into an apparently simple machine based system. In analogy to the human researcher described above, the part of the software that provides interaction with the human is presented to the subject as the blinded researcher, while the part of the software that defines the key is the third party. An example is the ABX test
ABX test
An ABX test is a method of comparing two kinds of sensory stimuli to identify detectable differences. A subject is presented with two known samples , and one unknown sample X, for three samples total. X is randomly selected from A and B, and the subject identifies X as being either A or B...
, where the human subject has to identify an unknown stimulus X as being either A or B.
In medicine
Double-blinding is relatively easy to achieve in drug studies, by formulating the investigational drug and the control (either a placeboPlacebo
A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...
or an established drug) to have identical appearance (color, taste, etc.). Patients are randomly assigned to the control or experimental group and given random numbers by a study coordinator, who also encodes the drugs with matching random numbers. Neither the patients nor the researchers monitoring the outcome know which patient is receiving which treatment, until the study is over and the random code is broken.
Effective blinding can be difficult to achieve where the treatment is notably effective (indeed, studies have been suspended in cases where the tested drug combinations were so effective that it was deemed unethical to continue withholding the findings from the control group, and the general population), or where the treatment is very distinctive in taste or has unusual side-effects that allow the researcher and/or the subject to guess which group they were assigned to. It is also difficult to use the double blind method to compare surgical and non-surgical interventions (although sham surgery
Sham surgery
Sham surgery is a faked operative intervention that omits the step thought to be therapeutically necessary. In controlled studies sham surgery is performed in the control population to assess the effect of the intervention under study by neutralizing the placebo effect and reducing bias...
, involving a simple incision, might be ethically permitted). A good clinical protocol will foresee these potential problems to ensure blinding is as effective as possible. It has also been argued that even in a double-blind experiment, general attitudes of the experimenter such as skepticism or enthusiasm towards the tested procedure can be subconsciously transferred to the test subjects.
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...
practitioners prefer blinded randomised controlled trials
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...
(RCTs), where that is a possible experimental design. These are high on the hierarchy of evidence; only a meta analysis of several well designed RCTs is considered more reliable.
In physics
Modern nuclear physicsNuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
and particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
experiments often involve large numbers of data analysts working together to extract quantitative data from complex datasets. In particular, the analysts want to report accurate systematic error
Systematic error
Systematic errors are biases in measurement which lead to the situation where the mean of many separate measurements differs significantly from the actual value of the measured attribute. All measurements are prone to systematic errors, often of several different types...
estimates for all of their measurements; this is difficult or impossible if one of the errors is observer bias. To remove this bias, the experimenters devise blind analysis techniques, where the experimental result is hidden from the analysts until they've agreed—based on properties of the data set other than the final value—that the analysis techniques are fixed.
One example of a blind analysis occurs in neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
experiments, like the Sudbury Neutrino Observatory
Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a neutrino observatory located 6,800 feet underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 1999,...
, where the experimenters wish to report the total number N of neutrinos seen. The experimenters have preexisting expectations about what this number should be, and these expectations must not be allowed to bias the analysis. Therefore, the experimenters are allowed to see an unknown fraction f of the dataset. They use these data to understand the backgrounds, signal-detection efficiencies, detector resolutions, etc.. However, since no one knows the "blinding fraction" f, no one has preexisting expectations about the meaningless neutrino count N' = N x f in the visible data; therefore, the analysis does not introduce any bias into the final number N which is reported. Another blinding scheme is used in B meson
B meson
B mesons are mesons composed of a bottom quark or bottom antiquark and either an up , down , strange or charm quark . The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark's short lifetime...
analyses in experiments like BaBar
Babar
Babar means Lion. Babar may refer to:Names* Babur , 16th-century ruler of Indian subcontinent and founder of the Mughal Empire* Babar Luck, musician from England...
and CDF
CDF
-Mathematics, science, and computers:* .cdf , the ANDI/netCDF mass spectrometry data interchange file format* Channel Definition Format, an XML standard* Charging Data Function, in the Session Initiation Protocol standard...
; here, the crucial experimental parameter is a correlation between certain particle energies and decay times—which require an extremely complex and painstaking analysis—and particle charge signs, which are fairly trivial to measure. Analysts are allowed to work with all of the energy and decay data, but are forbidden from seeing the sign of the charge, and thus are unable to see the correlation (if any). At the end of the experiment, the correct charge signs are revealed; the analysis software is run once (with no subjective human intervention), and the resulting numbers are published. Searches for rare events, like electron neutrinos in MiniBooNE
MiniBooNE
MiniBooNE is an experiment at Fermilab designed to observe neutrino oscillations . A neutrino beam consisting primarily of muon neutrinos is directed at a detector filled with 800 tons of mineral oil and lined with 1,280 photomultiplier tubes...
or proton decay in Super-Kamiokande
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...
, require a different class of blinding schemes.
The "hidden" part of the experiment—the fraction f for SNO, the charge-sign database for CDF—is usually called the "blindness box". At the end of the analysis period, one is allowed to "unblind the data" and "open the box".
In forensics
In a police photo lineup, an officer shows a group of photos to a witness or crime victim and asks him or her to pick out the suspect. This is basically a single-blind test of the witness' memory, and may be subject to subtle or overt influence by the officer. There is a growing movement in law enforcement to move to a double blind procedure in which the officer who shows the photos to the witness does not know which photo is of the suspect.External links
- “control group study“ – The Skeptic’s Dictionary – More on why double blind is important.
- PharmaSchool JargonBuster Clinical Trial Terminology Dictionary