Multidimensional item response theory models yielded good fit and reliable scores for the Short Form-12 questionnaire

Objectives: To propose a multidimensional item response theory (MIRT) scoring system for the Short Form 12 (SF-12) with good psychometric properties in terms of fit and reliability.

Study design and settings: Two models, indicating physical (PCS) and mental component summary (MCS) dimensions, were fitted to SF-12 data from the European Study of the Epidemiology of Mental Disorders, a representative sample from European adult general population (n=21,425; response rate=61.2%). Goodness of fit, information, reliability, and agreement of individual scores were compared with the classical SF-12 and RAND-12 algorithms.

Results: The bidimensional response process (BRP) model, where all items are indicators of both dimensions, yielded the best fit (root mean square error of approximation=0.057, comparative fit index=0.95, and Tucker-Lewis index=0.94), and highly agreed with PCS and MCS scores from the SF-12 (intraclass correlation coefficients of 0.92 and 0.88, respectively) and RAND-12 (0.88 and 0.95). Regarding reliability, the BRP yielded 0.75 and 0.77 (PCS and MCS, respectively), greater than SF-12 (0.65 and 0.66) and RAND-12 (0.65 and 0.67). As indicated by scale linking, MIRT scores can be interpreted similarly to the classical scores.

Conclusion: The MIRT models showed a clear construct structure for the PCS and MCS dimensions, defined by functional and role limitation content. Results support the use of SF-12 MIRT-based scores as a valid and reliable option to assess health status.