Second, since we are interested in comparing changes within subjects, we want to subtract each subject's response while taking Motrin from the equivalent response while taking placebo drug. Because different subjects received the drugs in different order, we need to be careful how we calculate these difference values. In STATA, we can use: gen painmx diff=(painmx 4-painmx 2) * (-1) ^(drg ord) [For group1 (drg_ord=1) Motrin is taken first and then Placebo, but for group 2 Placebo is taken first and then Motrin. By adding a multiplier (-1)(drg_ord), the difference will be calculated the same way for two groups]