Purpose: The SD-16 is a 16-item instrument assessing social distress in cancer patients. In addition to work underlining the scale's reliability, validity and clinical utility, recent research has also determined minimally important differences (MID) for the instrument. The aim of this study was to assess whether item locations remained stable over time, in order to support the longitudinal use of the instrument.
Methods: Data were collated from cancer patients (N = 123) who had completed the SD-16 at four time points (baseline, 6 months, 1 and 2 years). The Partial Credit Model (Rasch analysis) was applied to the data and item fit evaluated against published criteria. Measurement invariance of item location and person measures over time was assessed using analysis of variance.
Results: Good item fit was found with the exception of one item ("work"), which demonstrated misfit at two time points. No statistically significant differences were found for item locations. As anticipated, there were changes over time in patients' scores, particularly in the first 6 months.
Conclusions: The instrument can be used longitudinally to accurately assess changes in cancer patients' social distress.