Discriminant and convergent validity of a subjective quality-of-life instrument aimed at high content validity for schizophrenic persons.

OBJECTIVE: To analyse whether a subjective quality-of-life (QoL) instrument (QLiS-Quality-of-Life in Schizophrenia), developed on the basis of a large number of open-ended interviews with schizophrenic patients, has sufficient discriminant and convergent validity to justify its application as a schizophrenia-specific QoL instrument.
METHOD: The discriminant and convergent validity of the QLiS (comprising 12 subscales) was analysed in a cross-sectional study. Schizophrenic persons (n = 135) from different care settings were surveyed using the QLiS, the WHOQOL-Bref, the SWN and 7-point satisfaction items. Partial correlational analyses and regression analyses controlling for general life satisfaction were conducted comparing the QLiS subscales with those of the other instruments.
RESULTS: Positive correlation coefficients were found among all subscales of the QLiS and the other QoL instruments (WHOQOL-BREF from r = 0.29 to r = 0.72; SWN, r = 0.14 to r = 0.83; satisfaction scales, r = 0.18 to r = 0.69). One QLiS subscale (cognitive functioning) was shown to be empirically redundant (r>0.80) to the mental functioning subscale of the SWN. All other subscales proved to have unique variance. The non-QLiS QoL instruments only accounted for substantial amounts of variance (>20% after controlling for global life satisfaction) in the QLiS subscales leading a normal life, appreciation by others, appraisal of accommodation/housing and social contacts. DISCUSSION: Most of the QLiS subscales can be regarded as sufficiently distinct from other QoL instruments, and thus show evidence of discriminant and convergent validity.
CONCLUSION: A subjective QoL questionnaire with high content validity can provide additional empirical information about schizophrenics' QoL not accounted for by other common QoL instruments.