Incidence of something in a population, often expressed as a proportion or percentage. Base rates can affect the confidence in a decision. When base rates are relatively high, detection is much easier than in low base rate conditions. As an example, in medicine there is a much lower false positive rate with a diagnostic technique when the base rate of incidence is 50% than 0.1%. Similarly, in PDD it is much easier to find the one guilty individual in a tested population of two (50% base rate) than the 1 guilty person among 1,000 suspects (0.1% base rate). Statements of PDD validity are incomplete unless they also identify the characteristics of the tested population. The base rate problem is not unique to PDD but is also found in all other diagnostic tests. See: Gastwirth (1987); Kircher & Raskin (1987); Murphy (1987).