Rasch Scaling of a Screening Instrument

Abstract

The ERIraos Checklist (CL) is a screening instrument for assessing psychosis risk. Measuring CL data on a Rasch scale means that this scale locates individuals on a dimension of “proximity to psychosis onset” according to their current prodromal status. The probabilistic Rasch model leads to interval (difference) scales. The CL data from the German Research Network on Schizophrenia (GRNS) study were analyzed using the Rasch program Winsteps. All item measures based on data from different patient groups were consistent with the Rasch model. Examples demonstrate how item parameters were comparable in different subgroups and in patients in the early and late prodrome. The CL is a simple assessment tool fulfilling the requirements of a Rasch scale. This guarantees good psychometric properties, such as a high reliability and internal validity, and yields a measure of the construct “proximity to psychosis onset” on a difference scale.