I want to calculate effect sizes for running a meta-analysis. I am interessted in the difference between two groups after a treatement. In the papers included for the meta-analysis I often find the mean and standard deviation for each group, as well as (for example) t-values. In this case, the estimated effect size for the study varies - depending if I use the means and standard deviations for calculation of the t-test.
Which one would be the appropriate estimate - the one calculated based on means or the one calculated based on the t-value?