For all sample size calculation, alpha = 0.05 (i.e the confidence level =95%), and the power is 80% (i.e. beta = 0.2).

We are going to study the difference in proportions of two independent samples. We assume the proportions in these two samples are 0.30 (p1) and 0.20 (p2), respectively. Our hypothesis is H0: p1 = p2 vs Ha: p1 not equal to p2. To achieve a power of 80% to test the difference of 0.10, how many patients should we recruit in each group? The significance level is 0.05, and we assume attrition = 15%.