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Anecdotal evidence is an informal account of evidence in the form of an anecdote, or hearsay. The term is usually used in contrast to scientific evidence, especially evidence-based medicine, which are types of formal accounts. Anecdotal evidence is often unscientific because it cannot be investigated using the scientific method. Misuse of anecdotal evidence is a logical fallacy and is sometimes informally referred to as the "person who" fallacy ("I know a person who..."; "I know of a case where..." etc.) The problem with arguing based on anecdotal evidence is that anecdotal evidence is not necessarily typical; only statistical evidence can determine how typical something is.
When used in advertising or promotion of a product, service, or idea, anecdotal evidence is often called a testimonial. The term is also sometimes used in a legal context to describe certain kinds of testimony. Psychologists have found that people are more likely to remember notable examples than the typical example. This is why casinos usually try to draw attention to someone winning; this tends to make everyone else think that they are much more likely to win than they are. Another example is the fact that people are often afraid to fly after a major airline accident, even though flying is statistically similar in risk to driving.
In all forms of anecdotal evidence, testing its reliability by objective independent assessment may be in doubt. This is a consequence of the informal way the information is gathered, documented, presented, or any combination of the three. The term is often used to describe evidence for which there is an absence of documentation. This leaves verification dependent on the credibility of the party presenting the evidence.
In science, anecdotal evidence has been defined as:
- "information that is not based on facts or careful study" 
- "non-scientific observations or studies, which do not provide proof but may assist research efforts" 
- "reports or observations of usually unscientific observers" 
- "casual observations or indications rather than rigorous or scientific analysis" 
- "information passed along by word-of-mouth but not documented scientifically"
Anecdotal evidence can have varying degrees of formality. For instance, in medicine, published anecdotal evidence is called a case report, which is a more formalized type of evidence subjected to peer review.  Although such evidence is not regarded as scientific, it is sometimes regarded as an invitation to more rigorous scientific study of the phenomenon in question.  For instance, one study found that 35 of 47 anecdotal reports of side effects were later sustained as “clearly correct.” 
Researchers may use anecdotal evidence for suggesting new hypotheses, but never as supporting evidence.
Anecdotal evidence and faulty logic
Anecdotal evidence is often unscientific or pseudoscientific because various forms of cognitive bias may affect the collection or presentation of evidence. For instance, someone who claims to have had an encounter with a supernatural being or alien may present a very vivid story, but this is not falsifiable. This phenomenon can also happen to large groups of people through subjective validation.
Anecdotal evidence is also frequently misinterpreted via the availability heuristic, which leads to an overestimation of prevalence. Where a cause can be easily linked to an effect, people overestimate the likelihood of that the cause does have that effect (availability). In particular, vivid, emotionally-charged anecdotes seem more plausible, and are given greater weight. A related issue is that it is usually impossible to assess for every piece of anecdotal evidence, the rate of people not reporting that anecdotal evidence in the population.
A common way anecdotal evidence becomes unscientific is through fallacious reasoning such as the post hoc fallacy, the human tendency to assume that if one event happens after another, then the first must be the cause of the second. Another fallacy involves inductive reasoning. For instance, if an anecdote illustrates a desired conclusion rather than a logical conclusion, it is considered a faulty or hasty generalization.  For example, here is anecdotal evidence presented as proof of a desired conclusion:
- "There's abundant proof that God exists and is still performing miracles today. Just last week I read about a girl who was dying of cancer. Her whole family went to church and prayed for her, and she was cured."
Anecdotes like this are very powerful persuaders, but they don't prove anything in a scientific or logical sense.  The child may have become better anyway and this could be an example also of the regressive fallacy. Anecdotal evidence cannot be distinguished from placebo effects.  Only double-blind randomized placebo-controlled clinical trials can confirm a hypothesis.
- Anecdotal evidence, for example, is by definition less statistically reliable than other sorts of evidence, and explanations do not carry the weight of authority. But both anecdotal evidence and explanations may affect our understanding of a premise, and therefore influence our judgment. The relative strength of an explanation or an anecdote is usually a function of its clarity and applicability to the premise it is supporting. 
By contrast, in science and logic, the "relative strength of an explanation" is based upon its ability to be tested, proven to be due to the stated cause, and verified under neutral conditions in a manner that other researchers will agree has been performed competently, and can check for themselves.
Witness testimony is a common form of evidence in law, and law has mechanisms to test witness evidence for reliability or credibility. Legal processes for the taking and assessment of evidence are formalized. Some witness testimony could be described as anecdotal evidence, such as individual stories of harassment as part of a class action lawsuit. However, witness testimony can be tested and assessed for reliability. Examples of approaches to testing and assessment include the use of questioning, evidence of corroborating witnesses, documents, video and forensic evidence. Where a court lacks suitable means to test and assess testimony of a particular witness, such as the absence of forms of corroboration or substantiation it may afford that testimony limited or no "weight" when making a decision on the facts.
Scientific evidence as legal evidence
In certain situations, scientific evidence presented in court must also meet the legal requirements for evidence. For instance, in the United States, expert testimony of witnesses must meet the Daubert Standard. This ruling holds that before evidence is presented to witnesses by experts, the methodology must be "generally accepted" among scientists. In some situations, anecdotal evidence may meet this threshold (such as certain case reports which corroborate or refute other evidence).
Miller and Miller (2005) list five standards of proof, by level of evidence :
|Kind||Level of Evidence||Standard|
|Regulatory, Legal||Precautionary Principle|
|Legal - Civil||*||More likely than not|
|Legal - Civil||**||Clear and convincing|
|Legal - Criminal||***||Beyond a reasonable doubt|
Citing situations involving adverse drug reactions, Miller and Miller outline three events related to administration of the drug which can prove specific causation:
- challenge: the reaction occurs after the drug is given
- de-challenge: it resolves when the drug is discontinued
- re-challenge: the adverse event recurs when the drug is given a second time. (Cook County 2005)
Altman and Bland argue that the case report or statistical outlier cannot be dismissed as having no weight: "With rare and uncommonly occurring diseases, a nonsignificant finding in a randomized trial does not necessarily mean that there is no causal association between the agent in question and the disease." 
Miller and Miller conclude: "Most medical evidence does not meet the scientific standard of proof; and, as in law, it should be judged by a standard of proof appropriate to the fact or point in question. An 'anecdotal' case report can provide evidence of probative value, just like eyewitness testimony in a murder trial. And it can be similarly tested, by second opinions, re-examination, laboratory tests, and follow-up."
- Cambridge Advanced Learner's Diuctionary
- Jenicek M. "Clinical Case Reporting" in Evidence-Based Medicine. Oxford: Butterworth–Heinemann; 1999:117
- Vandenbroucke JP (2001). In Defense of Case Reports and Case Series. Annals of Internal Medicine. Vol. 134:4, 300-334
- Venning GR. Validity of anecdotal reports of suspected adverse drug reactions: the problem of false alarms. Br Med J (Clin Res Ed). 1982;284:249-52.PMID: 0006799125
- Thompson B. Fallacies.
- Logic via infidels.org
- Lee D (2005). Evaluating Medications and Supplement Products. via MedicineNet
- Graham R. Anecdotes.
- Miller, DW Jr, Miller, CG. On evidence, medical and legal. Journal of American Physicians and Surgeons Volume 10 Number 3, Fall 2005, 70-75.
- Altman DG, Bland M. Absence of evidence is not evidence of absence. British Medical Journal, 1995;311:485
- Overall Assessment of Approach and Analysis of the Law Commission Report Ministry of Economic Development, New Zealand
- Legal Terms & Definitions Sprenger & Lang, Attorneys
- Judge certifies Wal-Mart class action lawsuit MSBN, June 22, 2004
- Second Generation Disparity Study Final Report City of Phoenix study
- "Anecdotal Evidence" from a course in Critical thinking at Santa Rosa Junior College.
- Lindsay, Don. Anecdotal Evidence, via don-lindsay-archive.org
- Carroll, Robert Todd. Anecdotal (testimonial) evidence, from the Skeptic's Dictionary.
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