I have 5 timepoints (week 0, 2, 6, 12, 26) and the change from Baseline (BL) at week 12 is the variable interested. To work out the sample size for a future trial I would like to estimate the SD from a data set (N=400). Snce the original data are highly skewed the change from BL was log transformed (taking the ratio of visit xx/visit 0). I ran the following sas code to get the SE for each visit and then thought multiply by sqrt of n at that visit would give me the required SD? PROC GLM Data=powa2 ; Class sub prot visit ; Model logratio = base visit visit*base / solution ; LSMeans visit/ cl alpha=0.05; Run; quit; Can I use the SE for log transformed means for visit=12? Another way I am thinking is to run the model is to run model by visit and use the RMSE as the standard deviation for sample size calculation. Which way is better or is there any other way I should get the SDs for sample size cal.
... View more
