Table 2 Pre trial and during trial chlamydia testing numbers, percentages and positivity rates, by control (n = 23) and intervention (n = 20) groups Pre-trial period Women consulted N 2689 2662 Women tested N (%) 168 (6.2) 305 (11.5) Positive women N (%) 16 (9.5) 28 (9.2) Women consulted N 1792 1589 Women tested N (%) 157 (8.8) 213 (13.4) Trial period Positive women N (%) 15 (9.6) 7 (3.3) Women tested* Positive women** P value P value p<0.01 p = 0.06 p = 0.99 p = 0.01 * The difference in proportion of the number of women tested for chlamydia in the pre-trial and trial periods. ** The difference in proportion of the number of women who tested positive for chlamydia in the pre-trial and trial periods. Table 3 Adjusted odds ratios (AOR) for the effect of the intervention on the proportion tested. Female GPs Male GPs ≤ 44 years ≥ 45 years No Yes No Yes Control Intervention Pre-trial period Trial period Pre-trial period Trial period AOR (95%CI) 1.0 0.6 (0.4, 1.0) 1.0 0.5 (0.3, 0.9) 1.0 0.7 (0.4, 1.4) 1.0 1.2 (0.7, 2.3) 1.0 2.0 (1.1, 3.5) 1.5 (1.2, 1.9) 1.0 1.0 0.9 (0.6, 1.2) Control 16-24 yrs Intervention 16-24 yrs Variable GP gender Age group Study group Time period Postgraduate qualifications Knowledge of RACGP ‘Red Book’ guidelines Interaction of time with intervention # Adjusted for clustering at GP level There were a number of limitations to our study, most notably, that the observed change in testing was consid- erably lower than hypothesized. An imbalanced distribu- tion of variables was also evident at trial commencement, with GPs in the control group more likely to be younger and have a postgraduate qualifica- tion. GPs in the intervention group were also more than twice as likely to be testing for chlamydia at trial com- mencement. Although our multivariate analysis did adjust for these baseline differences, the lower than hypothesized effect size and the baseline imbalance in Bilardi et al. BMC Public Health 2010, 10:70 http://www.biomedcentral.com/1471-2458/10/70 Page 6 of 9 key confounders, did reduce our statistical power. The comparison of different pre-trial (12 months) and trial (6 months) time periods may not have allowed for sea- sonal changes in patient load and possible changes in testing frequence. In interpreting these results, it is important to note that one GP in the intervention group was testing very high numbers of young women before the trial commenced, accounting for nearly half the total number of chlamydia tests (44%) in the pre- trial period, but less than a quarter (23%) in the trial period. Despite thorough investigation into the decrease in this GP’s testing figures during the trial period, no practicable explanation could be found. If this GP is removed from the multivariate analysis, a greater increase in testing in the intervention group compared to the control group is evident suggesting a positive impact of the incentive payment (from OR = 0.9; 95% CI: 0.5, 1.6 to OR = 1.22; 95% CI: 0.8, 2.0). A further limitation of our study is that we did not provide GPs in the intervention group with ongoing feedback on their testing performance, nor were incen- tive payments made until the completion of the trial. It is likely that GPs forgot they were in the trial and con- sequently that they would receive payment for each eli- gible woman tested. The additional use of testing feedback on the number of tests performed during the trial period may have been useful in prompting beha- viour change in GPs [29]. It is likely that testing increased in both groups due to GPs raised awareness from the educational components of the study [30-32] - both the intervention and the control group received the same educational package prior to commencement in the trial. Educational inter- ventions as part of a multi-faceted intervention [33], and which combine strategies such as outreach visits and printed material have been shown to be effective in changing physician behaviour [30] without financial incentive. The significant decrease in positive chlamydia diagnosis in the intervention group is likely to be as a result of GPs testing higher numbers of low risk or asymptomatic women.