COMPARISON OF RASCH AND SUMMATED RATING-SCALES CONSTRUCTED FROM SF-36PHYSICAL FUNCTIONING ITEMS IN 7 COUNTRIES - RESULTS FROM THE IQOLA PROJECT

Rasch models for polytomous items were used to assess the scaling assu mptions and compare item response patterns in the IO item SF-36 physic al functioning scale (PF-IO) for general population respondents in Den mark, Germany, Italy, the Netherlands, Sweden, the United Kingdom, and the United States. The Rasch model of physical functioning developed in the United States was compared to models for other countries, and e ach country was compared to a multinational composite. Strong scale co ngruence across the seven countries was demonstrated; items that varie d between countries and from the composite may reflect unique cultural response patterns or differences in translation. Scoring algorithms b ased on the Rasch model for each country were superior to the current Likert scoring in tests of relative validity (RV) in discriminating am ong age groups in all countries. In relation to the Likert PF-10 scori ng (RV = 1.00), scores estimated using the Rasch rating scale model ac hieve a median RV of 1.31 (range: 1.01-1.59), while the Rasch partial credit model attained a median RV of 1.44 (range: 1.01-2.23). Rasch mo dels hold good potential for improving health status measures, estimat ing individual scores when responses to scale items are missing, and e quating scores across countries. (C) 1998 Elsevier Science Inc.