## “Type 1” slightly higher power than “Type 2” for the same adj. α [Two-Stage / GS Designs]

Hi BE-proff,

I agree with ElMaestro.

» 2) Why Method C is considered "better" for sponsors than Method B?

Unfortunately there is an „inflation” of letters denoting methods.
Therefore, I suggested* to use “Type 1” (B, E, …) and “Type 2” (C, D, C/D, F, …) instead.

“Type 1”

“Type 2”

In “Type 2” TSDs the conventional (unadjusted) α 0.05 may be used in the first stage (dependent on interim power). Hence, under certain conditions you have a decent chance to stop already in the first stage with no sample size penalty (due to the mandatory adjusted α in “Type 1” TSDs).

Potvin et al. recommended Method C over B due to its higher power. Examples (power by the noncentral t-approximation):

n1 CV (%)   B      C    12   10   0.97697 0.98858 24   20   0.88046 0.90882 36   30   0.83704 0.84676 48   40   0.82901 0.82838 60   50   0.82477 0.82405

Cheers,
Helmut Schütz

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