Can short PROMs support valid factor-based sub-scores? Example of COMQ-12 in chronic otitis media.

Purpose: Interpretable factor solutions for questionnaire instruments are typically taken as justification for use of factor-based sub-scores. They can indeed articulate content and construct validities of a total and components but do not guarantee criterion validity for clinical application. Our previous documentation of basic psychometric characteristics for a 12-item patient-reported outcome measure in adult chronic otitis media (COMQ-12) justified next appraising criterion validity of sub-scores.

Methods: On 246 cases at 1st clinic visit, we compared various classes of factor solution, concentrating on the best-fitting 3-factor ones as widely supported. Clinical data offered two independent measures as external criteria: binaural hearing (audiometric thresholds measured via audiometry) for evaluating 'Hearing' sub-score, and oto-microscopic findings for the 'Ear discharge symptoms' sub-score. As criterion for the total, and for semi-generic 'Activities/healthcare' sub-score, the generic Short Form-36 item set offered a widely used multi-item criterion measure.

Results: Factor model fit and parsimony again favoured a 3-factor solution for COMQ-12; however insufficient item support and the dominant 1st principal component of variation made sub-scoring problematic. The best solution was bi-factor, from which only the weighted total score met the declared convergent validity standard of r = 0.50. Two of the more specific sub-scores ('Ear discharge symptoms' and 'Hearing') correlated poorly with clinical findings and weighted binaural hearing thresholds.

Conclusion: The COMQ-12 total is acceptably content-valid for general clinical purposes, but the small item set, reflecting excessive pressure for brevity in clinical application, does not well support three criterion-valid factor-based scores. This distinction should be made explicit, and profile sub-scoring discouraged until good convergent and furthermore divergent criterion validities are shown.