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.
Highlights
Schizophrenia usually begins with an initial prodromal phase of several years duration
“difficult” items correspond to rarer symptoms, indicating growing proximity to psychosis onset
In the analysis presented here, only Item No 4 “disturbed body functions” exceeded this cutoff
Summary
Schizophrenia usually begins with an initial prodromal phase of several years duration. It is possible to demonstrate that the terminology of Rasch scaling is applicable to the association between symptom presence and stage of disease development for the purpose of measuring “proximity to psychosis onset” by symptoms of growing severity. If this conception is correct, the ERIraos Checklist (CL) should fulfill the requirements of the Rasch test model. Such scales are characterized by absolute measurement units of 1 (= logits) and an arbitrary zero-point, which is usually defined by an item with a solution probability of 50% This probabilistic test model is consistent with the fact that human behavior is not normally deterministic, but influenced by random factors. This attribute of the Rasch model is called “specific objectivity.”
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