Release Message:  Two independent events become conditionally dependent (negatively dependent) given that at least one of them occurs. Authored by Joseph Berkson.

Description:  Berkson's paradox or Berkson's fallacy is a result in conditional probability and statistics which is counterintuitive for some people, and hence a veridical paradox. It is a complicating factor arising in statistical tests of proportions. Specifically, it arises when there is an ascertainment bias inherent in a study design. It is often described in the fields of medical statistics or biostatistics, as in the original description of the problem by Joseph Berkson.