1) Why do we run group-sequential trials in drug development?
2) How does the effect we power at relate to the effect(s) needed to stop?
3) Is stopping such a trial early (like the one for the Pfizer vaccine) "cheating"?
So group-sequential designs reduce expected number of events needed and provide valid inference in reasonable cases.
This all within the framework of hypothesis testing, as required by Health Authority guidelines.
I hope this thread is useful. Comments welcome!
The end.