I'm not a mathematician which is why I asked the q

I'm not a mathematician which is why I asked the question. Originally, they considered interim at 50. I get the argument that 50 is too small a sample size. However, is there going to be that much difference between 180 and 195...?

Again, two more weeks of enrollment to hit 195 over 180 equals 14,000 people dead. So forgive the urgency while perhaps *slightly* compromising on the statistical sample size. Is there a reason 195 was chosen in the first place? Is that a mathematical number which makes the data easier to evaluate?