The PDQ-Carer: development and validation of a summary index score

Introduction: The PDQ-Carer is a 29 item measure of health related quality of life for use with carers of people with Parkinson's disease. The original development of the instrument identified four domains. This study examined the possibility that the four domains could reasonably be summed to provide a single summary index score.

Methods: The PDQ-Carer was administered in a postal survey of patients and carers registered with local branches of Parkinson's UK. Data from the four domains of the PDQ-Carer was subjected to higher order factor analysis. Reliability and validity of the summary index were assessed.

Results: A total of 236 carer questionnaires were returned, a response rate of 61%. The mean age of the sample was 68.2 years (SD 9.49; range 25-89 years); 63.5% females, 21.3% males. Higher order principle components factor analysis produced one factor, accounting for 85.5% of the variance. The newly derived single index was found to be both internally reliable (alpha = 0.94) and valid through correlations with the eight domains of the SF-36 (all p < 0.001).

Discussion: The analyses undertaken here indicate that data from the PDQ-Carer can be presented in summary form. The index will provide an overall indication of the impact of caring. Furthermore, the single index reduces the number of statistical comparisons, and hence the role of chance, when exploring data from the PDQ-Carer.