Psychometric properties of the PsychoSomatic problems scale: A Rasch analysis on adolescent data

The PsychoSomatic Problems (PSP)-scale is built upon eight items intended to tap information about psychosomatic problems among schoolchildren and adolescents in general populations. The purpose of the study is to analyse the psychometric properties of the PSP-scale by means of the Rasch model, with a focus on the operating characteristics of the items. Cross-sectional adolescent data collected in Sweden at six points in time between 1988 and 2005 are used for the analysis. In all more than 15,000 students aged 15-16 are included in the analysis. Data were examined with respect to invariance across the latent trait, Differential Item Functioning (DIF), item categorisation and unidimensionality. The results show that the PSP-scale adequately meets measurement criteria of invariance and proper categorisation of the items. Also the targeting is good and the reliability is high. Since the scale works invariantly across years of investigation it is appropriate for re-current monitoring of psychosomatic health complaints in general populations of adolescents. Taking DIF into account through principles of equating provides a scale that shows no statistically significant signs of gender-DIF enabling invariant comparisons also between boys and girls.