1. the naturally occurring frequency of a phenomenon in a population. An example is the percentage of students at a particular college who have major depressive disorder. This rate is often contrasted with the rate of the phenomenon under the influence of some changed condition in order to determine the degree to which the change influences the phenomenon. 2. in personnel psychology, the percentage of job applicants who would succeed in the job if all were hired. Base rates are used to measure the effectiveness of employment tests.