Background: Social difficulties may add to the psychological burden experienced by cancer patients. Therefore identifying social difficulties in routine oncology practice may help prevent or alleviate distress. The Social Difficulties Inventory (SDI) is a short questionnaire developed for assessing social difficulties in cancer patients. Although well-validated, not enough is known about the clinical meaning and utility of the instrument or whether the items can be meaningfully summed to form a summary index of "Social Distress".
Purpose: To determine, using Rasch analysis, whether the SDI could be used as a summary index of social distress specifically examining three fundamental criteria: item fit, unidimensionality and item invariance.
Methods: The Partial Credit Model was applied to a heterogeneous sample of cancer patients (n = 609) who had completed the SDI.
Results: Five items were identified as misfitting (infit mean square > or = 1.3 and standardised t-statistic > or = 2) and excluded from the subsequent analysis. The remaining items formed a unidimensional interval scale with no additional factors identified in a principal components analysis of the residuals. No differential item functioning was observed for age, gender, diagnosis, extent of disease or social deprivation. The 16-item SDI can be summed to produce an overall index of social distress, facilitating routine identification of social difficulties. Subsequent work is needed to evaluate whether the instrument is able to identify patients with high levels of social distress requiring intervention.