Clark -
Rancherho pointed out a while back that DNDN used Haybittle-Peto for interim alpha allocation in D9901. That might remain the same for the original IMPACT SPA. In that case, regardless of how many interims or when, the interim alpha remains a constant at .01.
On the other hand, they could have changed the alpha spending function to O'Brien-Fleming. This is "a big assumption". But, if true, the effect of lowering the final trigger and raising the interim one at the same time is to increase the information fraction (interim/final). In turn, this would raise the nominal alpha for the interim look without affecting that of the final too much. For example, according to OBF, if the interim is 230 and the final is 304, then the interim alpha would be .02 while the final alpha remains at .044. Although the sum of the alphas is larger than .05, this does not violate the preservation of the family alpha since the two analysis share the same set of events in the interim.
If we focus only on the data, lowering the final trigger will hurt the final p value since the 56 events taken out are the longer lasting events. So this move would make sense only if they see a substantial benefit in making the interim better. The PR was vague but its use of the term "alpha spending function" instead "alpha value" would make the above scenario not too tenuous.
If, as you thought, that this was all about finance and the alpha spending function remains Haybittle-Peto or whatever not OBF, then all doors are open. Let's see what they say tomorrow before deciding what is what.
