The SIP68: an abbreviated sickness impact profile for disability outcomes research

The Sickness Impact Profile (SIP) is one of the most widely recognized generic health status instruments, but its length has often left it out of consideration for outcomes research. We assess a short alternative, the Sickness Impact Profile 68 (SIP68), for retest and proxy reliability, validity, and scaling properties, in a population of adults with disability (PWD). For convergent validity, the SIP68 was compared to the instrumental activities of daily living (IADLs), activities of daily living (ADLs) and the short-form 36 (SF-36). We completed 398 interviews with PWD, 131 index-proxy sets, and 40 retests. Retest intraclass correlations were above 0.75 for all scales and dimensions except the physical dimension (0.61). Proxy reliability ranged from 0.26 (psychological autonomy and communication) to 0.85 (somatic autonomy). Correlation between the SIP68 and SIP was 0.94 overall; between the SIP68 and similar scales of the SF-36 correlations was moderate, and highest for physical health scales. We repeated the SIP68 development factor analysis and reproduced a structure of the full SIP that included 65 of SIP68 items. However, 36 additional items were retained that are not part of the SIP68. Overall, the SIP68 shows promise for use as a disability outcomes tool.