Evidence of internal construct validity of SOC-13 total score, for use in hand therapy

Purpose The objective was to examine whether the 13-item Sense of coherence scale (SOC-13) can be reported as a unidimensional interval-scale metric, when new approaches based on the Rasch model to address local item dependency are applied, and to determine whether an interval-scale scoring can be made available. Methods Data were derived from two samples of patients with hand-related disorders (merged n = 915). Rasch analyses of the SOC data were conducted using item-level analysis and a testlet approach. Results Initial item-level analysis of the SOC-13 confirmed previous findings of misfit to the Rasch model. In resolving local dependency by constructing three testlets, which corresponded to the three components of the SOC construct, fit to the Rasch model (chi(2)(df) = 43.11 (27), p = 0.163) and unidimensionality of the SOC-13 could be established. A transformation table was successfully created to convert the SOC-13 raw ordinal score to corresponding Rasch interval-scaled values. Conclusions The results of this study indicate that data obtained by the SOC-13 can be regarded as essentially unidimensional, and an interval-scale transformation table of the SOC-13 total scores was developed, for use in clinical practice and research on coping resources in patients with hand-related disorders.