Dear Sir/Madam I have a question about the simulated normal population. I am writing a review for Cochrane which involves meta-analyses. For these I need the mean difference and SD between the pre and posttreatment performance of drugs vs placebo. However, often these data are not given and it is usually not possible to get the real data from the authors. Calculating the mean difference = post-pretreatment performance Howver the SD of that mean difference is calculated as the square root of the sum of the variance of pre and posttreatment performance As I have the actual data of two studies, this allowed me to make comparisons. The estimated SD (using the formula) often overestimates the real SD sometimes by more than a factor 3. This varied whether the SD was small (overestimation less) or larger (overestimation went up). So I thought that I could use the simulated normal population in R and put in the mean and SD and N to then calculate the mean difference and SD. I used only 3 data points but the factor by which the simulated SD overestimated the actual SD seemed constant at 2.2 which at least is an improvement over the first formula. This is a serious problem as it increases type II errors. Do you have any idea how I can solve it ? Many regards Dr. E. Hogervorst University of Oxford, OPTIMA Radcliffe Infirmary Trust, Woodstock Rd, Oxford OX26HE tel 00 44 (0) 1865 228512 -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request@stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._