Abstract
The validity of scores derived from an educational or psychological testing situation determines the accuracy and appropriateness of inferences made about an examinee based on his/her test score. Person fit assessment provides a framework for assessing the conformity of a test score to a given measurement model or to a group of examinees as an indicator of validity/invalidity of the test score. This paper presents a brief overview of person fit assessment, the effect of person misfit on ability estimation, the PerFit R package as a powerful tool for person fit assessment, and two practical examples on how to use PerFit for person fit assessment.