We base the sample size calculations on a simple comparison of the post intervention mean referral qualities of the intervention arm and the control arm. All calculations are for 80% power and alpha-level 0.05, and relate to standardized differences. All results and plots were obtained using R. In this study we have fixed CPD groups with an average size of 8 associated GPs. We plan to invite all groups in the south county, meaning that the maximum number of groups is 58. The total number of referrals registered and scored can be higher by increasing the study period. The correlation of observations within each CPD group will reduce the power of the study, thus the number of observations required will be inflated by a factor related to the intraclass correlation coefficient (ICC). Some recent studies have reported on the ICC for antibiotics prescribing habits in these CPD groups; Flottorp et al (2002)found ICC=0.085, whereas Gjelstad et al found 0.013 and 0.061(2013). We anticipate that the qualities of referrals may be even more correlated than antibiotics prescriptions (at least on the GP level), thus we allow for ICCs from 0.05 to 0.15. Following Hemming et al (2011), we find that for a given number of CPD groups, there is a limit to the difference possible to detect, even for an infinite number of observations per group. Thus, we will likely not be able to detect "small" differences (standardized differences of 0.2 or smaller by Cohen's definition). Aiming for a sample size that enables us to detect a difference of 0.4 between the intervention and the control arm, we find that the number of observations required per CPD group depends on the number of groups included in the study. As can be seen, with smaller number of groups included in the study, the number of required observations per GP becomes very large in the "worst case" scenario. Thus, some effort should go into recruiting a maximal number of groups. Varying cluster sizes and drop-outs will reduce the power (Hemming et al., 2011), thus we plan to randomize 40 CPD groups, 20 in each group.