The example above is for two groups of subjects, but you should also use the concept of effect size when looking at changes in the mean as a result of an experiment. For example, the above two bars could represent muscle mass before and after treatment with anabolic steroids. In this case you use the SD of the pre scores only to standardize the effect. (I'd figured this out years ago, but until Oct 06 I missed a mistaken assertion on this page that you average the pre and post SDs. Sorry about that.) Some people think mistakenly that you should use the SD of the change scores to standardize effects in experiments. If you have a control group as well, you use the SD of all the pre scores, and you subtract the change in the control group from the change in the experimental group to get the magnitude of the experimental effect.