Elaine Rodger and Stephen Watkins ('Variations enigma', features, pages 20-23, September) found wide variation in the organisation and structure and some outputs of practices in the health authorities participating in the benchmarking project.

They did not take account of the registered patient population characteristics or the geographic variations in practice resourcing.

General practices in inner cities have limited premises and staffing resources for service delivery, and the incentives in the 1990 GP contract have not led to expected improvements in service delivery in deprived areas.

Research evidence also shows that organisation and structure affect service delivery. For example, patients registered with single handed practices are more likely to have acute hospital admissions for asthma in east London, and better organised practices are more likely to achieve a higher cervical screening coverage.

Studies have demonstrated the association between poverty and higher hospital admission rates, and GP referral rates. Therefore characteristics of the patient population have an important influence on the quality and quantity of primary care services.

Unlike acute hospital and, more recently, community services, general medical services funding is not weighted for deprivation. It would be interesting to see if the authors' analysis showed any relationship between deprivation and the HAs' benchmark. The effect of deprivation and level of GMS funding need to be controlled before targeted clinical indicators can be used for benchmarking.

Dr Jeannette Naish Senior lecturer Professor YH Carter Head of department Department of general practice and primary care St Bartholomew's and Royal London School of Medicine and Dentistry