Power curve of the required sample-size per treatment arm. The curve is represented as a function of willingness-to-pay (WTP) for a range of increases in costs (delta Costs) when a treatment strategy is not only more effective but also more costly. The presented +sample-size is unadjusted for intra-cluster correlation. A minimally important change in patient utility (EQ-5D) has been defined as 0.074 point. With 150 patients per treatment strategy we are able to detect at least a change of 0.06 points by net health benefit analysis between the arms with a SD = 0.175 EQ-5D points (baseline data SMASHING-project: SD = 0.17) and a SD of €1000 for costs (SD = €816, usual care strategy) and an increase in costs of €250 (delta Costs) when a treatment strategy is not only more effective but also more costly, for a willingness-to-pay (WTP) of €30K (alpha = 0.05, one-sided, beta = 0.20, one-sided, rho costs-effects = 0). With 40 clusters (general practices) per arm and assuming an intra-cluster correlation of 0.01, 0.07 and 0.11 the number of patients per cluster is 4, 5, and 6, and the total number of patients is 480, 600 and 720, respectively.
Honkoop et al. BMC Pulmonary Medicine 2011 11:53 doi:10.1186/1471-2466-11-53