Thanks Kevin. I thought the time t is at the end of follow-up (length of follow-up)?
John --- On Thu, 5/6/10, Kevin E. Thorpe <kevin.tho...@utoronto.ca> wrote: > From: Kevin E. Thorpe <kevin.tho...@utoronto.ca> > Subject: Re: [R] sample size for survival curves > To: "array chip" <arrayprof...@yahoo.com> > Cc: r-help@r-project.org > Date: Thursday, May 6, 2010, 8:20 PM > 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.
