The Base-Rate Bias

The tendency to ignore statistics and focus on the particulars.

Charts and numbers bore and intimidate most of us. We're data phobic, prefer the warm fuzzies of the human touch and testimonials to hard facts. The word "probability" makes us squeamish, feel dumb. Nonetheless, we need to know that when judging a situation―for instance, diagnosing a patient's disease―there are two often types of information:

Type #1  Data about the frequency of the disease occurring in a large population.
Type #2  Specific information about the patient: results of tests and an examination.

When people have both types of information, they tend to make judgments based entirely upon Type 2 information, leaving out the statistics. It's best to consider both types of information because there is always some possibility that an observation or test may be wrong. 

Source: Amos Tversky & Daniel Kahneman, "Evidential Impact of Base Rates", in Judgment Under Uncertainty: Heuristics and Biases, Kahneman, Paul Slovic, and Tversky, editors (1985), pp. 153-160.