The context of measuring disability: does it matter whether capability or performance is measured?

This study assesses the differences between two methods of conceptually framing physical disability questions, using two scenarios (capability and performance). The relationship between capability and performance was explored on the basis of the literature and empirically tested by administering two versions of the Activities Scale for Kids (ASK) to 28 physically disabled children. The capability version asked children what they "could do," whereas the performance version asked what they "did do." Capability was found to exceed performance (p < 0.001) by approximately 18%. The difference may relate to a difference in environmental contexts between the two versions, with performance reflecting abilities in usual (or real life) circumstances and capability reflecting abilities in a defined situation apart from real life. Researchers must, therefore, consider carefully the environmental circumstances in which they wish to evaluate outcomes, and use this information to decide whether to measure capability, performance, or both.