Evaluation of the Social Participation Questionnaire in adult patients with depressive symptoms using Rasch analysis.

Purpose: The aim of this study was to explore the psychometric properties of the 22-item Social Participation Questionnaire (SPQ).
Methods: The SPQ was administered to 789 adult primary care patients with depressive symptoms. As the items were intended to be summed together to provide total score, Rasch analysis (partial credit model) was applied to assess the overall fit of the model, individual item fit, differential item functioning (DIF), targeting of persons, response dependency, unidimensionality and person separation.
Results: To improve the scale's fit, it was necessary to re-score the response format. Two items demonstrated some DIF for gender and eight items showed DIF for age. To support the assumption of unidimensionality post hoc principal component analysis was performed. The analysis showed two subtests of the residuals with positive and negative loadings, but the person estimates derived from these two subtests were not statistically different to that derived from all items taken together. The response dependence between two items was identified; however, the magnitude of difficulty was very small. Although the questionnaire appeared to have insufficient items to assess the full spectrum of informal social contact, the SPQ was reasonably well targeted.
Conclusion: The SPQ is a promising questionnaire for the measurement of social participation although it could benefit from the inclusion of further items to measure informal social contact. This study found support for the internal validity, internal consistency reliability, and unidimensionality. A future study will investigate whether targeting can be improved when additional items are included.