Hi, maybe this is rather a question for statisticians. Suppose I have two groups of gene expression values, say group A and B with 10 samples each (20 in total). Performing a t-test I would choose the one with unequal variance, thus welch approximation. Usually the number of degrees of freedom (DF) in this case seems a little bit more complicated than with the equal variance version where I would substract 2 from the 20 (DF=18). The t.test package performs this calculation automatically. Now, digging in literature I still have the feeling, that I don't really understand what HAPPENS when I just combine the easy DF version with the welch approximation. Of course I get different pvalues and the DF will be equal for each gene, although the variances might be different. But what is the REAL consequence? Will the pvalues be unreliable by definition and why? regards Benjamin -- Benjamin Otto Universitaetsklinikum Eppendorf Hamburg Institut fuer Klinische Chemie Martinistrasse 52 20246 Hamburg