Establishing the minimal number of items for a responsive, valid, health-related quality of life instrument.

Reducing the number of items in a health-related quality of life instrument will enhance efficiency. However, it is important to maintain measurement properties. We determined the effect of reducing items from each domain (dyspnea, fatigue, emotion and mastery) of the 20-item Chronic Respiratory Questionnaire (CRQ). Three randomized trials of respiratory rehabilitation provided data. We removed items one at a time from each domain in three orders: by item impact, item responsiveness, and at random. Responsiveness, test-retest reliability and construct validity were evaluated at each step. Responsiveness and reliability, evaluated by intraclass correlation coefficients (ICC), were reduced marginally as the number of items was reduced to two items per domain. The deterioration was greatest when reducing from two items to one. To detect a particular effect, sample size would increase by about 10% when reducing the number of items in a domain to 2. Construct validity showed a more marked deterioration. Reducing to two items per domain would maintain responsiveness and reliability of the CRQ at an acceptable level, with a trade-off of reduced construct validity and increase in sample size requirements.