Measuring patients' views: a bifactor model of distinct patient-reported outcomes in psychosis.

BACKGROUND: Patient-reported outcomes (PROs) are widely used for evaluating the care of patients with psychosis. Previous studies have reported a considerable overlap in the information captured by measures designed to assess different outcomes. This may impair the validity of PROs and makes an a priori choice of the most appropriate measure difficult when assessing treatment benefits for patients. We aimed to investigate the extent to which four widely established PROs [subjective quality of life (SQOL), needs for care, treatment satisfaction and the therapeutic relationship] provide distinct information independent from this overlap.
METHOD: Analyses, based on item response modelling, were conducted on measures of SQOL, needs for care, treatment satisfaction and the therapeutic relationship in two large samples of patients with psychosis.
RESULTS: In both samples, a bifactor model matched the data best, suggesting sufficiently strong concept factors to allow for four distinct PRO scales. These were independent from overlap across measures due to a general appraisal tendency of patients for positive or negative ratings and shared domain content. The overlap partially impaired the ability of items to discriminate precisely between patients from lower and higher PRO levels. We found that widely used sum scores were strongly affected by the general appraisal tendency.
CONCLUSIONS: Four widely established PROs can provide distinct information independent from overlap across measures. The findings may inform the use and further development of PROs in the evaluation of treatments for psychosis.