Variability within general practitioner prescribing over time

We have described intra-general practitioner (GP) prescribing variability over time in terms of volume, cost and average item cost of prescription items, within New Zealand general practice.

Longitudinal data over the financial years 1992-94 were studied for two GP samples. Prescription data for a regional sample of 305 GPs were obtained for the first six months (January to June) from the New Zealand pharmaceutical pricing office, Health Benefits Limited. Prescription data from a second national sample of 74 GPs were obtained from the PreMeC prescription analysis (PAS) database of GPs who had participated in three consecutive September to December prescription analyses. The coefficient of variation was used to measure the intra-GP variability over time in total prescription cost, volume of prescription items and average prescription item cost.

The median intra-GP variability over time for the regional GP sample, based on reimbursement data, was 9% in total cost, 9% in total volume and 5% in average item cost. The median intra-GP variability in the national sample was very similar to the regional sample when based on reimbursement data, but when PAS data were used the variability was 16% in total cost, 17% in total volume and 8% average item cost.

The year-on-year, intra-GP variability for cost was 9%, for volume 9-10% and for average item cost 5-6%. Pharmaceutical budget estimates should reflect year-to-year intra-GP prescribing variability of the order of 9%.