Fig. 2

Contour plots showing futility stopping probabilities for a treatment difference of -4 (in favour of the control group) for the fixed rate recruitment and the uniform correlation model at (a) the first interim (\(p_{1}\)) and (b) the second interim analysis (\(p_{2}\)) as functions the correlation \(\alpha\) and the timing of the first interim analysis (\(t_{1}\); in 3 monthly base unit, such that for instance \(t_{1}=5\) corresponds to 15 months)