I have two treatment modalities (Drug A and PDT) and had observed that both in combination could yield a synergistic effect but however am not sure on how to prove it through statistics. Let's say these are my groups, I have a control untreated group, Drug A-only, PDT-only, and Combination. In the combination group, the protocol is that the cells were exposed to Drug A then immediately exposed to PDT. Do you have any ideas?