PerFit: An R Package for Person-Fit Analysis in IRT

Checking the validity of test scores is important in both educational and psychological measurement. Person-fit analysis provides several statistics that help practitioners assessing whether individual item score vectors conform to a prespecified item response theory model or, alternatively, to a group of test takers. Software enabling easy access to most person-fit statistics was lacking up to now. The PerFit R package was written in order to fill in this void. A theoretical overview of relatively simple person-fit statistics is provided. A practical guide showing how the main functions of PerFit can be used is also given. Both numerical and graphical tools are described and illustrated using examples. The goal is to show how person-fit statistics can be easily applied to testing of questionnaire data.