Re-scaling the Spanish version of the Sickness Impact Profile: an opportunity for the assessment of cross-cultural equivalence.

The Sickness Impact Profile (SIP), a behaviour-based measure of health status developed in the US, was adapted into Spanish. The aim of this study was to obtain the item weights of the Spanish version, to compare these weights with those of the original instrument, and to assess cross-cultural differences that may affect scale equivalence. Following the original work, weights for the Spanish items were obtained using the equal appearing intervals methodology in two groups of judges: 25 health professionals and 120 consumers. Correlation between professionals' and consumers' item weights was 0.93, thus data were combined for further analysis. Correlations between Spanish and American item weights was 0.89 (p < 0.001). Among each SIP category, correlations ranged from 0.61 to 0.99. Cultural differences were found among social items belonging to the social interaction category. While specific cultural differences exist when specific item weights were compared, overall cultural differences were not relevant. For practical purposes, the American and Spanish versions of the SIP should be considered cross-culturally equivalent.