Dear List, I have an experiment trying to evaluate two treatment for mice. I have 3 normal strains, two treatment and two time points. The goal is to compare t2 vs t1 Day14 vs Day0. All these mice considered normal. So I can create factor such as day0_t1, day0_t2, day14_t1, and day14_t2. and make contrasts, such as day0_t2-day0_t1, day14_t2-day14_t1, day14_t2-day14_t1... But how can I include strain information into the comparison? Since each strain's data will be correlated? Thank you. Xiaokuan Strain Day Treatment B6 0 t1 B6 14 t1 B6 0 t2 B6 14 t2 Balbc 0 t1 Balbc 14 t1 Balbc 0 t2 J129 0 t1 J129 0 t2 J129 14 t2 [[alternative HTML version deleted]]