Why anecdotal evidence is important

Anecdotal evidence is not proof. Wikipedia says:

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 ergo propter 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 drinking water cures cancer. Just last week I read about a girl who was dying of cancer. After drinking water she was cured.”

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”. Anecdotes often refer to the exception, rather than the rule: “Anecdotes are useless precisely because they may point to idiosyncratic responses.” Even when many anecdotes are collected to prove a point, “The plural of anecdote is not data.” (Roger Brinner)

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).

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:

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.

That is the case against anecdotal evidence. It makes it sound like personal experience and news you have heard is useless at best, and damaging at worst. But, let me show you how anecdotal evidence can be important.

The most important statement a scientist can make is “Huh, that doesn’t make sense. That shouldn’t have happened.” It is a sign post that new understanding and maybe even pushing the frontiers of science lies ahead. Science is made up of falsifiable statements about the universe. And to falsifiy, all you need is one counter example. Anecdotal evidence can be the early warning that a scientific theory has a problem. The smart scientist should explore further to discover what must be going on; to find the scientific evidence that proves what the anecdotal evidence hints at.

Serendipity is the accidental but fortunate discovery of something while looking at or thinking about something unrelated. Discovery itself most often starts with simple observation and leaps of thought to new ideas for understanding. But it is not proof.  The proof may take much hard work to create, but the inspiration is essential. Without it knowledge doesn’t advance.

The discovery of penicillin is a famous example where anecdotal evidence proves its value:

The improbable chain of events that led Alexander Fleming to discover penicillin in 1928 is the stuff of which scientific myths are made. Fleming, a young Scottish research scientist with a profitable side practice treating the syphilis infections of prominent London artists, was pursuing his pet theory — that his own nasal mucus had antibacterial effects — when he left a culture plate smeared with Staphylococcus bacteria on his lab bench while he went on a two-week holiday.When he returned, he noticed a clear halo surrounding the yellow-green growth of a mold that had accidentally contaminated the plate. Unknown to him, a spore of a rare variant called Penicillium notatum had drifted in from a mycology lab one floor below. Luck would have it that Fleming had decided not to store his culture in a warm incubator, and that London was then hit by a cold spell, giving the mold a chance to grow. Later, as the temperature rose, the Staphylococcus bacteria grew like a lawn, covering the entire plate — except for the area surrounding the moldy contaminant. Seeing that halo was Fleming’s “Eureka” moment, an instant of great personal insight and deductive reasoning. He correctly deduced that the mold must have released a substance that inhibited the growth of the bacteria.

It was a discovery that would change the course of history. The active ingredient in that mold, which Fleming named penicillin, turned out to be an infection-fighting agent of enormous potency. When it was finally recognized for what it was — the most efficacious life-saving drug in the world — penicillin would alter forever the treatment of bacterial infections. By the middle of the century, Fleming’s discovery had spawned a huge pharmaceutical industry, churning out synthetic penicillins that would conquer some of mankind’s most ancient scourges, including syphilis, gangrene and tuberculosis.

Via Time Magazine

Wikipedia has an extensive collection of historical examples of serendipity that shows the importance of anecdotal evidence. You don’t have to read them all, but look through them for an anecdotal example of the importance of anecdotal evidence. They are sorted into categories (Chemisty, Pharmacology, Medicine and biology, Physics and astronomy, Inventions) :


  • The German chemist Friedrich August Kekulé von Stradonitz having a reverie of Ourobouros, a snake forming a circle, leading to his solution of the closed chemical structure of cyclic compounds, such as benzene.
  • Lysergic Acid Diethylamide (or LSD) by Albert Hofmann, who found this potent hallucinogen while trying to find medically useful derivatives in ergot, a fungus growing on wheat.
  • Gelignite by Alfred Nobel, when he accidentally mixed collodium (gun cotton) with nitroglycerin
  • Polymethylene by Hans von Pechmann, who prepared it by accident in 1898 while heating diazomethane
  • Low density polyethylene by Eric Fawcett and Reginald Gibson at the ICI works in Northwich, England. It was the first industrially practical polyethylene synthesis and was discovered (again by accident) in 1933
  • Silly Putty by James Wright, on the way to solving another problem: finding a rubber substitute for the United States during World War II.
  • Chemical synthesis of urea, by Friedrich Woehler. He was attempting to produce ammonium cyanate by mixing potassium cyanate and ammonium chloride and got urea, the first organic chemical to be synthesised, often called the ‘Last Nail’ of the coffin of the Élan vital Theory
  • Pittacal, the first synthetic dyestuff, by Carl Ludwig Reichenbach. The dark blue dye appeared on wooden posts painted with creosote to drive away dogs who urinated on them.
  • Mauve, the first aniline dye, by William Henry Perkin. At the age of 18, he was attempting to create artificial quinine. An unexpected residue caught his eye, which turned out to be the first aniline dye—specifically, mauveine, sometimes called aniline purple.
  • Racemization, by Louis Pasteur. While investigating the properties of sodium ammonium tartrate he was able to separate for the first time the two optical isomers of the salt. His luck was twofold: it is the only racemate salt to have this property, and the room temperature that day was slightly below the point of separation.
  • Teflon, by Roy J. Plunkett, who was trying to develop a new gas for refrigeration and got a slick substance instead, which was used first for lubrication of machine parts
  • Cyanoacrylate-based Superglue (a.k.a. Krazy Glue) was accidentally twice discovered by Dr. Harry Coover, first when he was developing a clear plastic for gunsights and later, when he was trying to develop a heat-resistant polymer for jet canopies.
  • Scotchgard, is a 3M brand of products used to protect fabric, furniture, and carpets from stains, was discovered accidentally in 1953 by Patsy Sherman. One of the compounds she was investigating as a rubber material that wouldn’t deteriorate when in contact with aircraft fuel spilled onto a tennis shoe and would not wash out; she then considered the spill as a protectant against spills.
  • Cellophane, a thin, transparent sheet made of regenerated cellulose, was developed in 1908 by Swiss chemist Jacques Brandenberger, as a material for covering stain-proof tablecloth.
  • The chemical element helium. British chemist William Ramsay isolated helium while looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, noticed a bright-yellow spectral line that matched the D3 line observed in the spectrum of the Sun.
  • The chemical element Iodine was discovered by Bernard Courtois in 1811, when he was trying to remove residues with strong acid from the bottom of his saltpeter production plant which used seaweed ashes as a prime material.
  • Polycarbonates, a kind of clear hard plastic
  • The synthetic polymer celluloid was discovered by British chemist and metallurgist Alexander Parkes in 1856, after observing that a solid residue remained after evaporation of the solvent from photographic collodion. Celluloid can be described as the first plastic used for making solid objects (the first ones being billiard balls, substituting for expensive ivory).
  • Rayon, the first synthetic silk, was discovered by French chemist Hilaire de Chardonnet, an assistant to Louis Pasteur. He spilled a bottle of collodion and found later that he could draw thin strands from the evaporated viscous liquid.
  • The possibility of synthesizing indigo, a natural dye extracted from a plant with the same name, was discovered by a chemist named Sapper who was heating coal tar when he accidentally broke a thermometer whose mercury content acted as a catalyst to produce phthalic anhydride, which could readily be converted into indigo.
  • The dye monastral blue was discovered in 1928 in Scotland, when chemist A. G. Dandridge heated a mixture of chemicals at high temperature in a sealed iron container. The iron of the container reacted with the mixture, producing some pigments called phthalocyanines. By substituting copper for iron he produced an even better pigment called ‘monastral blue’, which became the basis for many new coloring materials for paints, lacquers and printing inks.
  • Acesulfame, an artificial sweetener, was discovered accidentally in 1967 by Karl Claus at Hoechst AG.
  • Another sweetener, cyclamate, was discovered by graduate student Michael Sveda, when he smoked a cigarette accidentally contaminated with a compound he had recently synthesized.
  • Aspartame (NutraSweet) was accidentally discovered by G.D. Searle & Company chemist James M. Schlatter, who was trying to develop a test for an anti-ulcer drug.
  • Saccharin was accidentally discovered during research on coal tar derivatives.
  • Saran (plastic) was discovered when Ralph Wiley had trouble washing beakers used in development of a dry cleaning product. It was soon used to make plastic wrap.
  • A new blue pigment with almost perfect properties was discovered accidentally by scientists at Oregon State University after heating manganese oxide.[8]


  • Penicillin by Alexander Fleming. He failed to disinfect cultures of bacteria when leaving for his vacations, only to find them contaminated with Penicillium molds, which killed the bacteria. However, he had previously done extensive research into antibacterial substances.
  • The psychedelic effects of LSD by Albert Hofmann. A chemist, he unintentionally absorbed a small amount of it upon investigating its properties, and had the first acid trip in history, while cycling to his home in Switzerland; this is commemorated among LSD users annually as Bicycle Day.
  • 5-fluorouracil’s therapeutic action on actinic keratosis, was initially investigated for its anti-cancer actions
  • Minoxidil‘s action on baldness; originally it was an oral agent for treating hypertension. It was observed that bald patients treated with it grew hair too.
  • Viagra (sildenafil citrate), an anti-impotence drug. It was initially studied for use in hypertension and angina pectoris. Phase I clinical trials under the direction of Ian Osterloh suggested that the drug had little effect on angina, but that it could induce marked penile erections.
  • Retin-A anti-wrinkle action. It was a vitamin A derivative first used for treating acne. The accidental result in some older people was a reduction of wrinkles on the face
  • The libido-enhancing effect of l-dopa, a drug used for treating Parkinson’s disease. Older patients in a sanatorium had their long-lost interest in sex suddenly revived.
  • The first anti-psychotic drug, chlorpromazine, was discovered by French pharmacologist Henri Laborit. He wanted to add an anti-histaminic to a pharmacological combination to prevent surgical shock and noticed that patients treated with it were unusually calm before the operation.
  • The anti-cancer drug cisplatin was discovered by Barnett Rosenberg. He wanted to explore what he thought was an inhibitory effect of an electric field on the growth of bacteria. It was rather due to an electrolysis product of the platinum electrode he was using.
  • The anesthetic nitrous oxide (laughing gas). Initially well known for inducing altered behavior (hilarity), its properties were discovered when British chemist Humphry Davy tested the gas on himself and some of his friends, and soon realised that nitrous oxide considerably dulled the sensation of pain, even if the inhaler was still semi-conscious.
  • Mustine – a derivative of mustard gas (a chemical weapon), used for the treatment of some forms of cancer. In 1943, physicians noted that the white cell counts of US soldiers, accidentally exposed when a cache of mustard gas shells were bombed in Bari, Italy, decreased, and mustard gas was investigated as a therapy for Hodgkin’s lymphoma.
  • Prontosil, an antibiotic of the sulfa group was an azo dye. German chemists at Bayer had the wrong idea that selective chemical stains of bacteria would show specific antibacterial activity. Prontosil had it, but in fact it was due to another substance metabolised from it in the body, sulfanilimide.

Medicine and biology

Physics and astronomy

  • The quite possibly apocryphal story of Archimedes‘ prototypical cry of Eureka when he realised in the bathtub that a body’s displacement water allowed him to measure the weight-to-volume ratio of any irregularly shaped body, such as a gold crown.
  • Isaac Newton‘s famed apple falling from a tree, supposedly leading to his musings about the nature of gravitation.
  • Discovery of the planet Uranus by William Herschel. Herschel was looking for comets, and initially identified Uranus as a comet until he noticed the circularity of its orbit and its distance and suggested that it was a planet, the first one discovered since antiquity.
  • Infrared radiation, again by William Herschel, while investigating the temperature differences between different colors of visible light by dispersing sunlight into a spectrum using a glass prism. He put thermometers into the different visible colors where he expected a temperature increase, and one as a control to measure the ambient temperature in the dark region beyond the red end of the spectrum. The thermometer beyond the red unexpectedly showed a higher temperature than the others, showing that there was non-visible radiation beyond the red end of the visible spectrum.
  • The thermoelectric effect was discovered accidentally by Estonian physicist Thomas Seebeck in 1821, who found that a voltage developed between the two ends of a metal bar when it was submitted to a difference of temperature.
  • Electromagnetism, by Hans Christian Ørsted. While he was setting up his materials for a lecture, he noticed a compass needle deflecting from magnetic north when the electric current from the battery he was using was switched on and off.
  • Radioactivity, by Henri Becquerel. While trying to investigate phosphorescent materials using photographic plates, he stumbled upon uranium.
  • X rays, by Wilhelm Roentgen. Interested in investigating cathodic ray tubes, he noted that some fluorescent papers in his lab were illuminated at a distance although his apparatus had an opaque cover
  • S. N. Bose discovered Bose-Einstein statistics when a mathematical error surprisingly explained anomalous data.
  • The first demonstration of wave–particle duality during the Davisson–Germer experiment at Bell Labs after a leak in the vacuum system and attempts to recover from it unknowingly altered the crystal structure of the nickel target and led to the accidental experimental confirmation of the de Broglie hypothesis. Davisson went on to share the 1937 Nobel Prize in Physics for the discovery.
  • Cosmic Microwave Background Radiation, by Arno A. Penzias and Robert Woodrow Wilson. What they thought was excess thermal noise in their antenna at Bell Labs was due to the CMBR.
  • Cosmic gamma-ray bursts were discovered in the late 1960s by the US Vela satellites, which were built to detect nuclear tests in the Soviet Union
  • The rings of Uranus were discovered by astronomers James L. Elliot, Edward W. Dunham, and Douglas J. Mink on March 10, 1977. They planned to use the occultation of the star SAO 158687 by Uranus to study the planet’s atmosphere, but found that the star disappeared briefly from view five times both before and after it was eclipsed by the planet. They deduced that a system of narrow rings was present.[9]
  • Pluto‘s moon Charon was discovered by US astronomer James Christy in 1978. He was going to discard what he thought was a defective photographic plate of Pluto, when his Star Scan machine broke down. While it was being repaired he had time to study the plate again and discovered others in the archives with the same “defect” (a bulge in the planet’s image which was actually a large moon).
  • High-temperature superconductivity was discovered serendipitously by physicists Johannes Georg Bednorz and Karl Alexander Müller, ironically when they were searching for a material that would be a perfect electrical insulator (nonconducting). They won the 1987 Nobel Prize in Physics.
  • Metallic hydrogen was found accidentally in March 1996 by a group of scientists at Lawrence Livermore National Laboratory, after a 60-year search.
  • A new method to create black silicon was developed in the lab of Eric Mazur.


  • Discovery of the principle behind inkjet printers by a Canon engineer. After putting his hot soldering iron by accident on his pen, ink was ejected from the pen’s point a few moments later.
  • Vulcanization of rubber, by Charles Goodyear. He accidentally left a piece of rubber mixture with sulfur on a hot plate, and produced vulcanized rubber
  • Safety glass, by French scientist Edouard Benedictus. In 1903 he accidentally knocked a glass flask to the floor and observed that the broken pieces were held together by a liquid plastic that had evaporated and formed a thin film inside the flask.
  • Corn flakes and wheat flakes (Wheaties) were accidentally discovered by the Kelloggs brothers in 1898, when they left cooked wheat unattended for a day and tried to roll the mass, obtaining a flaky material instead of a sheet.
  • The microwave oven was invented by Percy Spencer while testing a magnetron for radar sets at Raytheon, he noticed that a peanut candy bar in his pocket had melted when exposed to radar waves.
  • Pyroceram (used to make Corningware, among other things) was invented by S. Donald Stookey, a chemist working for the Corning company, who noticed crystallization in an improperly cooled batch of tinted glass.
  • The Slinky was invented by US Navy engineer Richard T. James after he accidentally knocked a torsion spring off his work table and observed its unique motion.
  • Arthur Fry happened to attend a 3M college’s seminar on a new “low-tack” adhesive and, wanting to anchor his bookmarks in his hymnal at church, went on to invent Post-It Notes.
  • The chocolate chip cookie was invented through serendipity. Chocolate chip cookies were invented by Ruth Wakefield when she attempted to make chocolate drop cookies. She did not have the required chocolate so she broke up a candy bar and placed the chunks into the cookie mix. These chunks later morphed into what is now known as chocolate chip cookies.

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