Anecdotal evidence
The expression anecdotal evidence has two quite distinct meanings.
(1) Evidence in the form of an anecdote or hearsay is called anecdotal if there is doubt about its veracity: the evidence itself is considered untrustworthy or untrue.
(2) Evidence which may itself be true and verifiable is used to deduce a conclusion which does not follow from it, usually by generalising from an insufficient amount of evidence. For example "my grandfather smoked like a chimney and died healthy in a car crash at the age of 99" does not disprove the proposition that "smoking markedly increases the probability of cancer and heart disease at a relatively early age". In this case the evidence may itself be true, but does not warrant the conclusion.
In both cases the conclusion is unreliable; it might happen not to be untrue, but it doesn't follow from the "evidence".
Evidence can be anecdotal in both senses: "Goat yoghurt prolongs life: I heard that a man in a mountain village who ate only yoghurt lived to 120".
The term is often used in contrast to scientific evidence, such as evidence-based medicine, which are types of formal accounts. Some anecdotal evidence does not qualify as scientific evidence because its nature prevents it from being 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. Compare with hasty generalization).  Anecdotal evidence is not necessarily typical; statistical evidence can more accurately determine how typical something is.
When used in advertising or promotion of a product, service, or idea, anecdotal reports are often called a testimonial, banned in some jurisdictions. 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 typical examples.

# 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.

# Scientific context
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 the cause having 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:
More generally, a statistical correlation between things does not in itself prove that one causes the other (a causal link). A study found that television viewing was strongly correlated with sugar consumption, but this does not prove that viewing causes sugar intake (or viceversa).
Anecdotes like this do not prove anything.  In any case where some factor affects the probability of an outcome, rather than uniquely determining it, selected individual cases prove nothing; e.g. "my grandfather smoked 40 a day until he died at 90" and "my sister never went near anyone who smoked but died of lung cancer".
In medicine anecdotal evidence is also subject to placebo effects: it is well-established that a patient's (or doctor's) expectation can genuinely change the outcome of treatment. Only double-blind randomized placebo-controlled clinical trials can confirm a hypothesis about the effectiveness of a treatment independently of expectations.
Sites devoted to rhetoric  often give explanations along these lines:
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.

# 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 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 :
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."