[R] Binomial Power/Sample Size

[Bret Collier]

All,

I have been digging around in the help files and found bsamsize in 
Hmisc, but I am wondering if i am using it right.

So, here is the question:  given a binomial response (success/failure) 
for 2 groups (treatment/control) and I want to estimate the necessary 
sample size (n) to determine if the magnitude of the difference between 
treatments and controls is a 25% increase in success probability.

Pilot data indicated that treatment success was ~0.32, control success 
~0.09.  So, using bsamsize (code below), I am interested in determining 
what sample size (n) is needed such that I can detect a 25% change in 
success between treatments/controls.

I tried this but I can't shake the feeling I am doing something wrong,

 > power_b<-bsamsize(.25, .0, fraction =0.5, alpha=0.10, power=0.80)
 > power_b<-as.data.frame(round(power_b, digits=1))
 > power_b
    round(power_b, digits = 1)
n1                       20.6
n2                       20.6

Any suggestions on approaches, places I should have looked would be helpful,

TIA,
Bret
Texas A&M University


platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          6.0
year           2007
month          10
day            03
svn rev        43063
language       R
version.string R version 2.6.0 (2007-10-03)