The factorial structure and stability of the McGill Pain Questionnaire in patients experiencing oral mucositis following bone marrow transplantation

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Abstract
The McGill Pain Questionnaire (MPQ) (Melzack 1975) is an important assessment tool for multidimensional pain measurement in both clinical practice and research. Despite widespread acceptance, empirical analyses have not consistently verified the 3 a-priori factors that guided the subclass construction of the Pain Rating Index (PRI) of the MPQ. This study compared the a-priori model with 2 qualitatively different factor models in 191 patients with oral mucositis pain at 3 days and 10 days following bone marrow transplantation. A semantic model defined by Sensory Action, Sensory Evaluation, and Affective Evaluation factors of subclass descriptor content fit better than the a-priori model and a model positing a single general pain factor. The 3 semantic PRI factors were highly intercorrelated, with the sensory factors correlating more highly with an independent visual analogue (VAS) pain scale. Standardized factor regression coefficients between the two occasions of measurement ranged between 0.4 and 0.5. Mean factor change was greatest for Sensory Evaluation and lowest for Affective Evaluation. All analyses were conducted with the LISREL 7 structural equation modeling program. Although the factor analyses indicated an unambiguous ranking of PRI models according to statistical criteria, these theoretical results generalize poorly to simple scores formed by direct addition of the PRI subclasses. Summary scores can only approximate the unobserved factors and cannot retain the fine discriminations revealed by the theoretical factors. Psychometric considerations suggest that a single PRI total score will yield better practical measurement than any scoring rules based on multiple factors.