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T-test is comparing means of different groups, or compares the mean of one group to a specified value. You will need more subjects in each group if you would like to compare them. D. --- On Thu, 8/12/10, tiffany huang <[log in to unmask]> wrote: From: tiffany huang <[log in to unmask]> Subject: [SPM] paired-t-test To: [log in to unmask] Date: Thursday, August 12, 2010, 6:17 PM Dear SPMers, I am new to SPM and have a question about the paired-t-test in the 2nd level. I wanted to compare two con images from the same subject, so I entered con_0001.img and con_0002.img as a pair in the paired-t-test. However, all of the betas (below the design matrix) are gray, indicating that none of them is uniquely specified. Attached is a picture of it. I do not have this problem when I was building a design matrix for each subject in the 1st level. Is it saying that I can't do a paired-t-test for con_0001 and con_0002? I also tried different pairs for the same subject but I got the same thing, i.e. gray betas. Any hint would help. Thanks so much in advance. Tiffany