[R] query: sample size calculation

[P Ehlers]

ManuelPerera-Chang at fmc-ag.com wrote:

> Dear all,
> 
> I am doing something wrong.
> 
> I am trying to apply a formula for  sample size calculation as in the book
> "Design and Analysis of Clinical Trials", from Chow et all.
> 
> There a suggested sample size strategy uses the formula
> 
> 0.30/0.45=F(0.80,2n,2n)/F(0.025,2n,2n)
> 
> which gives n=96, as in the book.
> 
> (here F(alfa,k,n) is the upper (alfa)th quantile of an F distribution with
> k,n degrees of freedom)
> 
> I have been trying to get n=96 using the following code in R.
> 
> val<-rep(NA,200)
>  for (n in 1:200) {
>   val[n]<- qf(0.8, 2*n, 2*n,lower.tail = TRUE, log.p = FALSE)/qf(0.025,
> 2*n, 2*n,lower.tail = TRUE, log.p = FALSE);
>  }

Don't you want 'lower.tail = FALSE'?

Peter Ehlers

> 
> but val doesnot get any close to 0.30/0.45.
> 
> Thank you,
> 
> Manuel
> 
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