But if all participants use the same amount of soap, then differences in bacteria count among the three treatments cannot be due to the amount of soap used. But if all participants wash their hands fo...But if all participants use the same amount of soap, then differences in bacteria count among the three treatments cannot be due to the amount of soap used. But if all participants wash their hands for the same amount of time, then differences in bacteria count among the three treatments cannot be due to the amount of time participants washed their hands. This is what we mean when we say that the control works by stabilizing the impact of the confounding variable across the treatments.