The amount necessary to highlight a significant difference between the two groups of 30 % of consumption at 12months is 250 patients ( 50 GPs , 5 patients per GP ; risk = 5 % ; power = 90 % ; intra-cluster correlation coefficient = 0.2 ; Hawthorne effect = 15 % ; lost to follow-up rates for GPs = 10 % and for patients = 20 % ) .