Key points are not available for this paper at this time.
L. S. Freedman, D. J. Spiegelhalter, The Assessment of the Subjective Opinion and its Use in Relation to Stopping Rules for Clinical Trials, Journal of the Royal Statistical Society. Series D (The Statistician), Vol. 32, No. 1/2, Proceedings of the 1982 I.O.S. Annual Conference on Practical Bayesian Statistics (Mar. - Jun., 1983), pp. 153-160
Freedman et al. (Tue,) studied this question.