Anecdotal evidence

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Anecdotal evidence is an informal account of evidence in the form of an anecdote. 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.

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.

What constitutes anecdotal evidence is sometimes disputed on scientific or philosophical grounds.

Contents

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Introduction

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.

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Scientific context

In science, anecdotal evidence has been defined as:

  • "report of clinical experiences based in individual cases, rather than an organised investigation with appropriate controls, etc." [1]
  • "information that is not based on facts or careful study" [2]
  • "non-scientific observations or studies, which do not provide proof but may assist research efforts" [3]
  • "reports or observations of usually unscientific observers" [4]
  • "casual observations or indications rather than rigorous or scientific analysis" [5]
  • "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.

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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.

The most common way anecdotal evidence becomes unscientific is through a fallacy involving inductive reasoning. For instance, if an anecdote illustrates a desired conclusion rather than a logical conclusion, it is considered a faulty or hasty generalization. [6] 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 actually prove anything in a scientific or logical sense. [7] Anecdotal evidence cannot be distinguished from placebo effects. [8] Only double-blind randomized placebo-controlled clinical trials can confirm a hypothesis.

Sites devoted to rhetoric [9] often give explanations along these lines:

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. [10]
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Law

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 harrassment 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.

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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
Scientific **** Irrefutable

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." (Altman 1995)

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."

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References

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See also

he:ממצא אנקדוטלי

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