Measurement invariance of the 16-item social distress scale Adam B. Smith • Penny Wright • Peter Selby • Galina Velikova Accepted: 25 September 2010 / Published online: 5 November 2010 Ó Springer Science+Business Media B.V. 2010 Abstract 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. Keywords Social distress Á Rasch Á Longitudinal Á Cancer Introduction Cancer has an impact on the everyday lives of patients resulting in a range of social difficulties. These are expe- rienced by people of all ages, both sexes, at different stages of disease and from all socioeconomic backgrounds [1]. Social difficulties related to cancer and cancer treatments may add to the burden, psychological distress and uncer- tainty already experienced by patients and their families. The UK’s National Institute for Clinical Excellence (NICE) recommends that the social care needs of cancer patients are identified and addressed on an ongoing basis [2]. Once these issues are identified, the health care team may discuss them with the patients and, if appropriate, suggest interventions with the aim of alleviating the difficulty. However, there are barriers to routine assessment of social care needs in oncology practice, which include logistical problems not only of how to assess so many people at different time points, but also of finding an assessment tool that is brief, covers the range of social difficulties of importance to all cancer patients and that is easy to use and score. The twenty-one item Social Diffi- culties Inventory (SDI-21) was developed specifically for use in routine oncology practice as no other measure was identified which addressed the scope of everyday difficul- ties reported by cancer patients [1, 3, 4]. The SDI-21 covers a broad range of social difficulties, including caring for dependents, access to financial services and problems with work. A Rasch analysis of the instrument [5] has demon- strated that 16 of the 21 SDI-21 item scores may be summed to form a scale of social distress (SD-16). No differential item functioning was observed for the SD-16 across age groups, gender, stages of disease, disease sites and levels of deprivation. A score of 10 or above on SD-16 A. B. Smith (&) Centre for Health & Social Care, Leeds Institute of Health Sciences, University of Leeds, Leeds LS2 9JL, UK e-mail: a.b.smith@leeds.ac.uk P. Wright Á P. Selby Á G. Velikova Cancer Research—UK Psychosocial and Clinical Practice Research Group, St. James’s University Hospital, Leeds, UK 123 Qual Life Res (2011) 20:507–511 DOI 10.1007/s11136-010-9782-6