  In a review on the contribution of J.B.S. Haldane to the development of the
Bayes factor hypothesis test (arXiv:1511.08180), Etz and Wagenmakers focus on
Haldane's proposition of a mixture prior in a genetic example (Haldane 1932, A
note on inverse probability. Mathematical Proceedings of the Cambridge
Philosophical Society, 28, 55-61.). As Haldane never followed up on these
ideas, it is difficult to gauge his motivation and intentions. I argue that
contrary to Haldane's stated intention of replacing flat priors with more
reasonable assumptions, he actually chose in this example an unreasonable flat
prior. Considering the information available to Haldane, I derive a superior
prior and compare to Haldane's flat prior. Haldane's main intent with his
article seems to have been to explore the different parameter regions of the
binomial and the conjugate beta. Furthermore, I agree with Etz and Wagenmakers
that Haldane serendipitously adopted a mixture prior comprising a point mass
and smooth distribution in his genetic example.
