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To return to Anoop's original question, example 4.1 p108-109 of my book Cross-over Trials in Clinical Research (2nd edition), Wiley, 2002, illustrates the use of a permutation test on a cross-over trial with 12 subjects. As regards robustness, an interesting paper is that of Donald Preece, 'T is for trouble (and textbooks): a critique of some examples of the paired-samples t-test. The Statistician, 31, 169-195 (1982). Stephen ----- Original Message ---- From: Anoop Balachandran <[log in to unmask]> To: [log in to unmask] Sent: Sat, July 30, 2011 8:35:45 AM Subject: Sample Size Question Hi everyone, How valid is a study which uses a sample size of 12 with a within-subject design? Or how well can we extrapolate the conclusions to the population studied. I have heard that you need at least 30 per group to get a normal distribution. That been said, I have read that even numbers approaching 10 can get you close to a normal distribution. The study I am talking about measures muscle fiber size using a biopsy and also assess strength measurements. In the exercise field, we sill use P-values so they haven't reported the confidence intervals. Thanks!