You have to think about the hypothesis you are testing. This would be that groups A and B (including both the first set of 200 and second set of 100 people) hav

You have to think about the hypothesis you are testing. This would be that groups A and B (including both the first set of 200 and second set of 100 people) have been independently sampled from the same distribution. This hypothesis is used to calculate a prediction of the expected results. Instead your data is forced to consist of a sample where mean(A)-mean(B)=delta, where delta>0, for the first 200 people. Knowing that, would you make the same prediction about the final result (after the data from all 300 people is in)? It would be the same as getting data from all 300 people at once and adding delta to the first 200 in Group A. Clearly you have specifically created a deviation from the original hypothesis by design. Also, let me note I said nothing about validity. I don't consider testing a hypothesis different than the research hypothesis to be a valid scientific activity. Just because the null hypothesis is false does not mean your research hypothesis is accurate or useful, so it is pointless from a scientific perspective. To me, this is on the level of arguing whether the Holy Spirit proceeds from the Father, or the Father and the Son. The entire premise behind the discussion is flawed, but we can still discuss the proper application of reason/logic to the arguments flowing from this flawed premise.