Anecdotal evidence is evidence that comes from individual experience. For example if I have had a good experience growing a particular kind of plant in my garden and I tell other people, that provides anecdotal evidence that the plant will grow well in my local area.

Other important kinds of evidence are statistical evidence, scientific evidence and legal evidence.

Deciding how much weight to give to different kinds of evidence is important if you want to establish the truth about a particular issue. Often, of course, truths cannot be firmly established and then whatever evidence is available must simply be used to determine what is most likely to be true given the balance of probabilities.

The Problems that Bedevil Anecdotal Evidence

Jumping to conclusions

It is easy to believe that because something has happened once it will always happen. This is often not the case. Superstitions are often an outcome of unreliable assumptions.

Missing the point is common form of jumping to conclusion.

If something happens it is easy to ascribe the wrong reasons for the occurrence eg. If a cat falls sick after it is seen eating a fish, it might be believed that the fish is poisonous- even if it the sickness was caused by a disease no one was aware of.

Unreliable or Selective Memory

The person who relates the anecdote may forget key things.

Hearsay

Often anecdotes are related not by the person who experienced an event but by someone who has heard it related by someone else. Mistakes can creep into the account and questions posed by the listener cannot be answered.

Embroidery

A person who has reached a conclusion may embroider an account for a whole range of reasons. People may sensationalize a story so as to become the center of attention. Sometimes there are financial reasons for embroidery- a salesman may tell many anecdotes about his products which exaggerate their benefits.

Other Forms of Evidence

Scientific evidence is widely seen as the most reliable evidence in matters relating to the physical world. In a scientific study great efforts are made to eliminate the kinds of errors that bedevil anecdotal evidence. Observation, record keeping, care in reaching conclusions, and the design of experiments to prove or disprove any assertions made, are central to the scientific method. Statistical analysis is often used to determine how much confidence should be placed in the conclusions of a scientific paper.

Part of the scientific process is constant testing and re-testing of assertions by independent researchers. Gradually a body of evidence is built up.

Statistical Analysis

Statistical analysis uses certain mathematical formulae to establish connections, trends or correlations. Statistical evidence is never conclusive however.

In the nineteen sixties a body of statistical evidence was gradually built up which suggested a statistically significant correlation between smoking and likelihood of developing cancer. Tobacco companies argued that there was no direct evidence that smoking led to cancer despite the correlation. Subsequent scientific investigation and experiment discovered that were chemicals in cigarette smoke that caused cancer in animals. It is now accepted that smoking increases the risk of developing cancer.

http://en.wikipedia.org/wiki/Hypothesis_testing

Legal Evidence

An important part of legal evidence is spoken testimony. Testimony is similar to anecdote in that a witness relates his or her experience of an event. In a court of law, however, the weaknesses of anecdotal evidence are well known and all testimony is thoroughly examined. Testimony is tested by cross examination, by comparison to accounts from other witnesses, by logical analysis (eg does the account have an internal consistency) and by reference to physical and documentary evidence.

The reputation, skills and education of the witness may also be taken into account. On technical issues, more weight will be given to expert witnesses for instance. Any possible motivation for fabrication will be sought out. Deliberate deception is deterred by the law of perjury. Hearsay is not admitted in any legal system based on English common law,

Why Anecdotal Evidence will Always be Important

Scientific investigation and legal proceeding may be safer ways of establishing what is true and what is not but they are also time consuming and very expensive.

Anecdotal evidence and personal experience will always remain vital in decision making and creating a vision of the world.

Anecdotal evidence is also a great driver of events and history. Revolutions, wars, market crashes etc, don't grow out of scientific studies.

What is evidence in a court of law:

http://en.wikipedia.org/wiki/Evidence_(law)

What is considered valid scientific evidence:

http://en.wikipedia.org/wiki/Scientific_evidence