The significance of importance: an evaluation of Ferrans and Powers' Quality of Life Index.

Ferrans and Powers' Quality of Life Index (QLI) defines and assesses quality of life (QoL) in terms of importance-weighted life satisfaction. This study assessed the value of such weights and explored the relationship between weighted and unweighted (satisfaction only) scores and single-item rated overall life satisfaction (LS) and QoL. Data were collected by a postal survey to 81 Parkinson's disease patients (88% response rate). Correlations between weighted and unweighted QLI scores were >or=0.96, except for one subscale (r ( s ) = 0.85). Item non-response rates ranged between 4.2 and 45.1% and 1.4 and 38% for the weighted and unweighted QLI, respectively. Cronbach's alpha exceeded 0.7 for weighted and unweighted versions of two out of the four subscales and the total score. Scaling success rates were similar for weighted and unweighted scores and did not support the current subscale structure. Unexpectedly, weighted total scores correlated stronger with LS than with QoL, and unweighted scores displayed the opposite pattern. This study found no advantages by using importance-weighted satisfaction scores. The correlational pattern with overall LS and QoL challenges the QLI approach to QoL, although these observations may relate to the use of multiplicative item weights. This study has implications also beyond the QLI regarding, e.g., the use of multiplicative weights and the relationship between life satisfaction and QoL.