The removal of the treatment in a reversal design is essential for demonstrating causality. While a simple AB design cannot easily rule out the possibility that an extraneous variable caused the change in the dependent variable, a reversal design addresses this issue. If the dependent variable changes when the treatment is introduced and then reverts when the treatment is removed, it strongly indicates that the treatment caused the effect. This reversal process significantly improves the internal validity of the study, provided the treatment's effects are not permanent.