I brought up two potential problems that I see happening the scenario of both arms being put on V after interim analysis. At interim, DMC will compare the total of events in V group, let's call it (A) vs Placebo group, let's call it (B). If the rate of (B) = 5.0 and the rate is (A) is 4.0, RRR = 20%.
If what you predict occurs and both groups are given V, this will form a new group, let's call it (C). I don't understand how RRR AFTER interim could be calculated.
DMC compared (A) to (B) to calculate RRR before. Now we will have (C). The only way to calculate RRR after we have (C) is to compare (C) to (B) but I see problems which will lead to inaccuracies:
- Population in (C) will be older than in (B). So when comparing the two, it's not fair to AMRN because (C) will be compared to a younger population.
- One can say that population in (C) is much healthier than in (B) because no one in (C) has gotten an event thus far. I understand population is chosen randomly, but what's not random is that everyone in (C) did not have an event, so it's not a fair comparison to (B).