Chapman and Chapman (1971) studied the effect as it relates to psychodiagnostic signs. Their study showed that although projective testing is not helpful in the diagnosis of mental disorders, some psychologists continue to use such tests because of a perceived, illusory, correlation between test results and certain attributes. An example of a projective test is the "Draw a Person" test that asks patients to draw a person on a blank piece of paper. Some psychologists believe in a correlation between drawing a person with big eyes and paranoia. No such correlation exists, and when data that is deliberately uncorrelated is presented to college students they find the same illusory correlations that psychologists believe in.
This bias can be caused by, among other things, an event that stands out as unique. For example, "The only time I forget my pencil is when we have a test". This is most likely an illusory correlation. It could be caused by only a few other pencil-less tests, which stand out particularly well in memory.
2 See also
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