A base rate in psychology is the naturally occurring rate of something, such as depression, in a population without any outside influences. It is also sometimes referred to as a prior probability. The base rate can be used as a comparison to a population using a new treatment to determine its effect.
In psychology, researchers often experience something known as the "base case fallacy" in their studies. This occurs when participants ignore general information in favor of individual information. For example, participants may ignore test scores and grades of students to determine a school's performance and instead focus on descriptive information about the students.