How should I calculate sample size for a clinical trial when I need a power of 80% and I predict an effect size to be at lest 0.5? do I need to estimate the mean of outcome as well?
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There is a free program called Gpower that is specifically designed to help you answer these kinds of questions. One of its features is that it computes "sample sizes for given effect sizes, alpha levels, and power values (a priori power analyses)".
If you were looking at the difference of two means you have enough but not for a linear model. The SAS proc glmpower will solve the problem for you when you have a general linear model. I think it can be used for mixed models as well as fixed effects models. I don't know whether the free software will do this. If you don't have SAS nQuery Advisor and Power and Precision are inexpensive sample size/power determination tools that you can use.