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Table 5 Empirical power and the percentage of trials stopping for futility and efficacy for sample size re-estimation using conditional power

From: Impact of misspecifying the distribution of a prognostic factor on power and sample size for testing treatment interactions in clinical trials

θ Planned distribution of the prognostic factor Planned n t Empirical power Percent stopping for futility Percent stopping for efficacy
Misspecification of the prognostic factor: -5%
5 10% 1,418 0.7324 16.58% 5.76%
20% 798 0.8410 10.34% 11.76%
30% 608 0.8616 9.02% 13.72%
40% 532 0.8754 8.24% 15.42%
50% 512 0.8868 7.64% 16.22%
15 10% 178 0.7246 17.14% 7.90%
20% 100 0.8232 12.26% 11.82%
30% 78 0.8736 8.94% 14.76%
40% 68 0.8936 7.72% 15.80%
50% 64 0.9010 7.10% 16.38%
Misspecification of the prognostic factor: -15%
5 10% 1,418 -- -- --
20% 798 0.5614 26.06% 3.5%
30% 608 0.7536 15.46% 7.3%
40% 532 0.8304 10.72% 10.80%
50% 512 0.8750 8.34% 14.32%
15 10% 178 -- -- --
20% 100 0.6026 22.68% 4.96%
30% 78 0.7634 15.76% 8.50%
40% 68 0.8412 10.78% 11.86%
50% 64 0.8734 9.32% 14.76%
Misspecification of the prognostic factor: +5%
5 10% 1,418 0.9528 3.10% 27.44%
20% 798 0.9194 5.58% 20.94%
30% 608 0.9064 6.26% 18.36%
40% 532 0.8898 7.18% 17.16%
50% 512 0.8868 7.52% 16.02%
15 10% 178 0.9476 3.94% 29.36%
20% 100 0.9194 5.70% 21.78%
30% 78 0.9204 5.94% 20.38%
40% 68 0.9146 6.36% 18.28%
50% 64 0.9056 6.96% 15.88%
Misspecification of the prognostic factor: +15%
5 10% 1,418 0.9868 0.94% 45.24%
20% 798 0.9494 3.38% 27.26%
30% 608 0.9186 5.34% 20.84%
40% 532 0.8990 6.98% 17.12%
50% 512 0.8822 8.00% 14.30%
15 10% 178 0.9902 0.78% 49.94%
20% 100 0.9584 3.18% 29.86%
30% 78 0.9412 4.32% 22.32%
40% 68 0.9108 6.56% 17.12%
50% 64 0.8816 8.76% 13.84%
  1. The margin of error based on the 99% confidence interval is 0.015.