Hi R-users, I want to calculate the sample size needed to carry out a 2-sample proprotion test. I have the hypotesized treatment probability of success (0.80), the hypotesized control probability of success (0.05), and also de proportion of the sample devoted to treated group (5%), (fraction=rho=0.05, n2/n1=19). Using the Hsmisc library, it seemss that I can use bsamsize (option 1) or samplesize.bin (option 2, alpha=0.05 or option 3 alpha=0.05/2, I am not sure after reading the help page) and I can use STATA (option 4). library(Hmisc) #OPTION 1: bsamsize(p1=0.8, p2=0.05, fraction=0.05, alpha=.05, power=.9) # n1 =2.09, n2=39.7, TOTAL=42 #OPTION 2: samplesize.bin(alpha=0.05, beta=0.9, pit=0.8, pic=0.05, rho=0.05) # n= 58, TOTAL= 58 #OPTION 3: samplesize.bin(alpha=0.025, beta=0.9, pit=0.8, pic=0.05, rho=0.05) # n= 72, TOTAL= 72 #OPTION 4: sampsi 0.8 0.05, p(0.90) a(0.05) r(19) # n1=4, n2 = 76 TOTAL=80 Can the method used produces the differences (42 vs 72 vs 80)? Can somebody give me hints about the possible reasons (asymptotic-exact distribution- continuity correction-my own error)? Which method would be recomended? Thanks a lot in advance, Berta