Does self-weighting of items enhance the performance of an oral health-related quality of life questionnaire?

Objectives: To determine if self-weighting of the items in an oral health-related quality of life questionnaire improves its psychometric properties. Methods: The Surgical Orthodontic Outcome Questionnaire (SOOQ) was designed to assess the oral health-related quality of life of individuals before and after corrective surgery. Each of its 33 'items' consists of two questions: a question concerning the frequency with which a given functional or psychosocial problem had been experienced and a 'weighting' question which asked about how much the individual was bothered by that problem. The questionnaire was completed by three groups of individuals: (i) pretreatment; (ii) immediate (i.e. 2-6 months) postsurgery and (iii) postsurgery (i.e. more than 2 years after surgery). Unweighted scale scores were obtained by summing the response codes to the frequency question and weighted scores by summing the products of the frequency and bother questions. These scores were calculated for the full questionnaire and a short form consisting of 15 items. The discriminative and correlational construct validity of these scores was compared along with internal consistency reliability. The sensitivity to change and longitudinal construct validity of unweighted and weighted scores was assessed in a simulated evaluative study in which pretreatment and postsurgery subjects were paired. Results: For both the long and short forms of the questionnaire, unweighted and weighted scores discriminated between the groups enrolled in the study. Correlations with a general health rating were similar, as were Cronbach's alpha values and test-retest reliabilities. The simulated evaluative study suggested no differences in sensitivity to change or longitudinal construct validity. When subscale scores were examined, there was a suggestion that weighting improved their reliability. Conclusions: Self-weighting of items did not substantially improve the performance of the SOOQ. Domain weights should be developed and tested to determine if they have an effect on its properties.