The "-glm_output option in randomise creates model estimates. The beta's are *_glm_pe_tstat1.nii.gz", where "pe" = "parameter estimates". We are usually interested in contrast parameter estimates ("cope"), so "_glm_cope_tstat1.nii.gz". The pe files will have one value (volume) per parameter in the model, whereas the cope files will have a single value corresponding to the contrast. For example, if you have group, age and sex, you will get files like below, where the "pe" files have three volumes each:
To view group differences:
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