Nonbinding futility rule [Two-Stage / GS Designs]

Dear Detlew,

» If I see it correctly that's called in Maurer et al. "futility rule can be applied in a nonbinding manner, ie, it can be used as guidance but must not necessarily be followed." (page 19, bottom)
» How to obtain a sample size number for the second stage if we want to do so?

The reference at page 19 actually refers to the CI futility criterion, but nevertheless the same argument should apply to the 'power of stage 1' criterion. Well, as I said: Formula (15), i.e. the formula for the estimated conditional target power is only defined if the power of stage 1, P(R1), is less than the overall power 1 - beta. That's a key feature of the equation. Therefore, in my opinion the only possibility is: if you want to be able to handle it in a nonbinding manner, then you have to go with conditional error rates only (i.e. you cannot use the estimated conditional target power as target power for calculation of n2). So, we would need to select ssr.conditional = "error".

Best regards,
Ben.

PS: Please don't shoot the messenger