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 (dy spnea, fatigue, emotion and mastery) of the 20-item Chronic Respiratory Que stionnaire (CRQ). Three randomized trials of respiratory rehabilitation pro vided 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. Resp onsiveness and reliability, evaluated by intraclass correlation coefficient s (ICC), were reduced marginally as the number of items was reduced to two items per domain. The deterioration was greatest when reducing from two ite ms to one. To detect a particular effect, sample size would increase by abo ut 10% when reducing the number of items in a domain to 2. Construct validi ty showed a more marked deterioration. Reducing to two items per domain wou ld maintain responsiveness and reliability of the CRQ at an acceptable leve l, with a trade-off of reduced construct validity and increase in sample si ze requirements. (C) 2001 Elsevier Science Inc. All rights reserved.