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anecdotal (testimonial) evidence
Testimonials and vivid anecdotes are one of the most popular and convincing forms of evidence presented for beliefs in the supernatural, paranormal, and pseudoscientific. Nevertheless, testimonials and anecdotes in such matters are of little value in establishing the probability of the claims they are put forth to support. Sincere and vivid accounts of one’s encounter with an angel or the Virgin Mary, an alien, a ghost, a Bigfoot, a child claiming to have lived before, purple auras around dying patients, a miraculous dowser, a levitating guru, or a psychic surgeon are of little value in establishing the reasonableness of believing in such matters.
Anecdotes are unreliable for various reasons. Stories are prone to contamination by beliefs, later experiences, feedback, selective attention to details, and so on. Most stories get distorted in the telling and the retelling. Events get exaggerated. Time sequences get confused. Details get muddled. Memories are imperfect and selective; they are often filled in after the fact. People misinterpret their experiences. Experiences are conditioned by biases, memories, and beliefs, so people's perceptions might not be accurate. Most people aren't expecting to be deceived, so they may not be aware of deceptions that others might engage in. Some people make up stories. Some stories are delusions. Sometimes events are inappropriately deemed psychic simply because they seem improbable when they might not be that improbable after all. In short, anecdotes are inherently problematic and are usually impossible to test for accuracy.
Thus, stories of personal experience with paranormal or supernatural events have little scientific value. If others cannot experience the same thing under the same conditions, then there will be no way to verify the experience. If there is no way to test the claim made, then there will be no way to tell if the experience was interpreted correctly. If others can experience the same thing, then it is possible to make a test of the testimonial and determine whether the claim based on it is worthy of belief. As parapsychologist Charles Tart once said after reporting an anecdote of a possibly paranormal event: “Let’s take this into the laboratory, where we can know exactly what conditions were. We don’t have to hear a story told years later and hope that it was accurate.” Dean Radin also noted that anecdotes aren't good proof of the paranormal because memory “is much more fallible than most people think” and eyewitness testimony “is easily distorted”(Radin 1997: 32).
Testimonials regarding paranormal experiences are of little use to science because selective thinking and self-deception must be controlled for in scientific observations. Most psychics and dowsers, for example, do not even realize that they need to do controlled tests of their powers to rule out the possibility that they are deceiving themselves. They are satisfied that their experiences provide them with enough positive feedback to justify the belief in their paranormal abilities. Controlled tests of psychics and dowsers would prove once and for all that they are not being selective in their evidence gathering. It is common for such people to remember their apparent successes and ignore or underplay their failures. Controlled tests can also determine whether other factors such as cheating might be involved.
If such testimonials are scientifically worthless, why are they so popular and why are they so convincing? There are several reasons. Testimonials are often vivid and detailed, making them appear credible. They are often made by enthusiastic people who seem trustworthy and honest, and who lack any reason to deceive us. They are often made by people with some semblance of authority, such as those who hold a Ph.D. in psychology or physics. To some extent, testimonials are believable because people want to believe them. Often, one anticipates with hope some new treatment or instruction. One’s testimonial is given soon after the experience while one’s mood is still elevated from the desire for a positive outcome. The experience and the testimonial it elicits are given more significance than they deserve.
Finally, it should be noted that testimonials are often used in many areas of life, including medical science, and that giving due consideration to such testimonials is considered wise, not foolish. A physician will use the testimonies of his or her patients to draw conclusions about certain medications or procedures. For example, a physician will take anecdotal evidence from a patient about a reaction to a new medication and use that information in deciding to adjust the prescribed dosage or to change the medication. This is quite reasonable. But the physician cannot be selective in listening to testimony, listening only to those claims that fit his or her own prejudices. To do so is to risk harming one’s patients. Nor should the average person be selective when listening to testimonials regarding some paranormal or occult experience.
See also ad hoc hypothesis, Occam's razor, cold reading, communal reinforcement, control study, placebo effect, post hoc fallacy, selective thinking, self-deception, subjective validation, and wishful thinking.
Alzheimer’s and Diet "Nita Scoggan ... is promoting herself as a 'health and happiness coach' and using her husband as anecdotal evidence of the power of her nutritional advice. The story is now being promoted by other low-carb gurus, including Jimmy Moore. The story is a great example of why anecdotes are so problematic."