We begin by randomly assigning half the subjects to the first testing room. The other half will be tested in the second room. Next, we randomly assign half the subjects in each room to the treatment condition A; the remining subjects will be in treatment condition B. Notice that we have not wiped out the differents between the two testing rooms; they are just as different as ever. However, the hope is that the effects of the rooms are the same, or balanced, for both treatment conditions. For every A subject tested in the green pink room, a B subject is also tested there.