Figure 1.

The type I error rate $P_{0,{\delta }_B}\left(R_A^{\prime }\right)$ to reject H _A as function of δ _B when applying the simultaneous stopping rule or separate stopping rule for boundaries v ₁,v ₂ satisfying ((2)) (dashed curves) or the simultaneous stopping rule for improved boundaries v1′,v2′ = v ₂ where v1′ solves ((4)) (solid curves) for O'Brien Fleming boundaries (left graph) and Pocock boundaries (right graph). No futility bound is applied (l ₁=−∞). The horizontal dashed lines show the nominal α level and the levels corresponding to v1′ and v ₁.