Effect of the intervention on imaging referral.

1No. Practices = 34 and No. GPs = 44; Total number of Medicare patients seen by GPs in intervention group = 77,716.2No. Practices = 37 and No. GPs = 40; Total number of Medicare patients seen by GPs in control group = 75,226.3Incident rate ratios estimated from negative binomial models fitted using GEEs with an exchangeable correlation structure and robust variance estimation to allow for clustering within general practices. For models where the estimated within cluster correlation was negative (indicated by*), the model was refitted assuming an independent correlation structure. All models adjusted for the design strata (number of GPs per practice, location of practice [metropolitan or rural/remote]) and confounders specified prior to undertaking the analysis (age of GP (years), years since GP graduated, special interest in LBP, practice method of billing [bulk bill or co-payment]).