array chip wrote:
Dear R users, I am not asking questions specifically on R, but I know there are many statistical experts here in the R community, so here it goes my questions:Freedman (1982) propose an approximation of sample size/power calculation based on log-rank test using the formula below (This is what nQuery does): (Z(1-α/side)+Z(power))^2*(hazard.ratio+1)^2 N = --------------------------------------------- (2-p1-p2)*(hazard.ratio-1)^2 Where Z is the standard normal cumulative distribution. p1 and p2 are the survival probability of the 2 groups at a given time, say t. As you can see, the sample size depends on the survival probabilities, p1 and p2. This is where my question lies. Let’s say we have 2 survival curves. I can choose p1 and p2 at time 1 year, and calculate a sample size. I can also choose p1 and p2 at time 5 years (still the same hazard ratio since the same 2 survival curves), and calculate a different sample size. How to interpret the 2 estimates of sample size? This problem doesn’t occur when we calculate the number of events required using this formula: 4*( Z(α/side)+Z(power))^2 -------------------------- (log(hazard.ratio))^2 Because number of events required only depends on hazard ratio. Thanks for any suggestions. John
As I recall, the survival probability used in Freedman is not at some arbitrary time of your choosing, but rather at the average length of follow-up time anticipated in the study.
Kevin -- Kevin E. Thorpe Biostatistician/Trialist, Knowledge Translation Program Assistant Professor, Dalla Lana School of Public Health University of Toronto email: kevin.tho...@utoronto.ca Tel: 416.864.5776 Fax: 416.864.3016 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
