Measurement properties of the 13-item sense of coherence scale using Rasch analysis

The sense of coherence (SOC) scale is widely used and has an extensive history in research. The psychometric properties of the SOC scale have been investigated using classic test theory, but modern test theory enables a more multifaceted investigation of the properties of the SOC scale. The aim of this study was to explore the measurement properties of the SOC scale using the Rasch measurement model. SOC questionnaires from a sample of 623 healthy adults were analysed using Rasch analysis. Aspects analysed were rating scale functioning, item fit, unidimensionality, differential item functioning (DIF), targeting, and reliability. Rating scale analysis showed that the seven scale steps were not utilized in the intended manner and that a shortening to five categories would be beneficial. Twelve out of the 13 items showed acceptable goodness-of-fit and 43 % of the variance was explained by the SOC dimension in the principal components analysis. There was no DIF between subgroups in the sample. The items were well targeted to the sample SOC level with no ceiling or floor effects. Item and person reliability were good and the person separation index was 2.05 indicating that the scale can separate three different levels of SOC, which corresponds well to its theoretical base. The SOC scale is generally well functioning; however, the three components of SOC seem to influence the PCA results. The scale would benefit from a reduction from seven to five scale steps, which would need to be investigated further.