- A decrease in events of 30% In a cluster randomized trial, the outcome of individuals within clusters is not independent. The sample size should be increased to take into account these effects. The required increase is influenced by both the ICC and the size of the cluster. The investigators must increase the standard sample size by an inflation factor equal to [(1 + (n-1) ρ)] where n is the average size of centers in our study , estimated to 3000 patients per year, ρ the intraclass correlation coefficient (ICC).