Hi everyone I have a question regarding to the analysis of gene expression by chip-array. When I am analyzing the ratio of two groups of samples by making average of all the samples in the each group, do I need to do log transformation for each sample and then to calculate average and the ratio or to calculate average and the ratio and then to do log transformation. For example, if I have in group1 the values: a,b and c and in group 2: d, e and f, do I need to do- log2[average(a,b,c)/average(d,e,f)] OR average(log2 a, log2 b, log2 c)/ average(log2 d, log2 e, log2 f)? If someone can help me with this question I will be very grateful.