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Dear SPM experts, This question might be a bit strange, but it has been bothering me for a while. I am doing VBM analysis; basically, I compare the differences of gray matter volume between groups. My design is the "two-sample t-test", in which the input scans are normalized and smoothed T2 maps, and I also model some non-interest covariates. What bothers me is that, most of the time when I conduct the analysis, the first column and second column which represent the group are normal as below. 1 0 1 0 1 0 0 1 0 1 0 1 However, sometimes when I process other subjects in another study with exactly the same procedure and parameter, the first two columns will become uneven number, an example is shown below. Furthermore, it does not happen when I specify the model, only happens after I hit "estimate". 1.05 0 1.05 0 1.05 0 0 0.95 0 0.95 0 0.95 I just can't figure out what caused the number difference. In my knowledge, the first two column should be always composed of 1 and 0. Can anyone tell me what is wrong and will that effect the accuracy of my analysis? A screenshot of my design matrix is attached. Thank you Yi-Shin