Hi R-users, I want to calculate the sample size needed to carry out a 2-sample proprotion test (with alfa=0.05, beta=0.8) 1.- probability of success in subpopulation A: 0.8 2.- probability of success in subpopulation B: 0.05 3.- percentage of population in subpopulation A = 5%, 4.- percentage of population in subpopulation B= 95% (the rest, A and B complementary) (Note that it is dificult to find people from subpopulation A) Using library(Hmisc) (or others if exist?), I would like to know which one of these three options (if any) should I use: # FIRST: bsamsize(p1=0.8, p2=0.05, fraction=0.05, alpha=.05, power=.8) # n1 n2 # 1.46 27.82 # SECOND: samplesize.bin(alpha=0.05, beta=0.8, pit=0.8, pic=0.05, rho=0.05) # n= 42 # THIRD: To calculate sample size assuming fraction=0.5 and then multiplying by 0.05: bsamsize(p1=0.8, p2=0.05, fraction=0.5, alpha=.05, power=.8) # n1 n2 # 5.47 5.47 # so that n=6*100/5=120 Taking into account the difficulty of obtaining the samples, which method would be recomended? Or in other words, how many people should I find, around 30-40 or 120? Thanks a lot in advance, Berta