The denominator for audit in general practice

Different denominators for morbidity studies were compared from two large studies in Britain. From the second national morbidity survey, data from 24 single-handed doctors showed a close correlation between the denominators 'persons consulting' and 'list size' (r greater than 0.9) in both years of the survey, but a weaker correlation between 'consultations' and 'list size' (r = 0.6). However, when examining rank order statistics for visiting and out-patient referral rates, it was immaterial for most doctors which denominator was chosen. Only for recorders with a consultation rate at the extremes of the range was the choice of denominator critical to the interpretation of the data. In the practice activity analysis study, based on 47 doctors and a mean of 284 consultations in two weeks, the correlation between 'persons consulting' and 'total consultations' was 0.99. Thus the number of consultations provided a satisfactory proxy for persons consulting in a two-week study. These results justify the use of 'consultations' over two weeks as a denominator in general practice audit in circumstances where rank order is appropriate for the interpretation of data.