Background: The ideal population size of healthcare commissioning organisations is not known.
Aim: To investigate whether there is a relationship between the size of commissioning organisations and how well they perform on a range of performance measures.
Design and setting: Cross-sectional, observational study of performance in all 152 primary care trusts (PCTs) in England.
Method: Comparison of PCT size against 36 indicators of commissioning performance, including measures of clinical and preventative effectiveness, patient centredness, access, cost, financial ability, and engagement.
Results: Fourteen of the 36 indicators have an unadjusted relationship (P<0.05) with size of the PCT. With 10 indicators, there was increasing quality with larger size. However, when population factors including deprivation, ethnicity, rurality, and age were included in the analysis, there was no relationship between size and performance for any measure.
Conclusion: There is no evidence to suggest that there is an optimum size for PCT performance. Observed variations in PCT performance with size were explained by the characteristics of the populations they served. These findings suggest that configuration of clinical commissioning groups should be geared towards producing organisations that can function effectively across their key responsibilities, rather than being based on the size of their population alone.