Experimental Design questions I have a couple of questions regarding the procedures in an experiment.  If I would like to test out two different drugs and the effect it has on the subject, why would it be a good idea to randomize the treatments (which drug is administered first and which one is administered second) and why would it be good to have a gap between administering the two treatments, ie.) should I give them two days apart, or a week, and why?
 A: In experimental design, you are attempting to control for the effects of influencing factors that you are not measuring (because you may not deem them unimportant and/or you are not interested in their influences at this stage). By randomising the order of treatments, you are 'controlling' for the influences of administering drug 1 followed by drug 2 and vice versa. There may be a nuanced biological/psychological effect of administering drugs in a particular order, which will influence your results and may hinder the interpretability of the experiment. Therefore, by randomising the order of treatments given, neither ordering is biased and any differences in subject performance after treatment can, more confidently, be assumed to be caused by the biological influences of the drug in isolation. 
Likewise, by having a gap between treatments you are 'controlling' for the interaction between the two drugs. This gap will be informed about the half-life of the drug etc. Any biological interaction between the drugs makes it a) potentially detrimental to the welfare of a subject and b) difficult to analyse the influence of a drug in isolation on biology. 
