Pretest sensitization refers to the potential or actuality of a pretreatment assessment's effect on subjects in an experiment. The term has different meanings in various disciplines. In medicine and biology, it typically refers to a physical reaction to an initial administration of a drug or regimen. In the social sciences, it has come to mean a cognitive or psychological change in a subject due to administration of a test or observation of the subject. Although the mechanism of sensitization is typically not investigated or even known, the effect on behavior or response has been long documented. This is referred to here as the pretest effect. This entry focuses on the research of this phenomenon, methods of evaluation, and related statistical considerations.