Dear list: I have a question about how to do expression microarray normalization when doing group comparisons. I have 40 individuals (20 cases, 20 controls) and each one has 3 mRNA chips at different time-points. Should I normalize all the chips (40x3) together (and the subset for each comparison) or if I am comparing cases_time1 vs controls_time1, or cases_time1 vs cases_time2, should I normalize only these 40 chips involved in this particular comparison? (I am working with Illumina chips) I am new in the expression analysis and I would appreciate any comments about advantages or disadvantages regardind these two approaches. Thank you very much! ANA BARBARA GARCIA Unidad de Genotipado y Diagnóstico Genético Valencia Clinical Hospital Research Foundation CIBER of Diabetes and Metabolic Diseases [[alternative HTML version deleted]]