Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 44 / 2 / f / Solution

Multiplying the likelihood in part (d) by the Haldane prior gives $p(\varphi \mid y)\propto {\varphi }^{s-1}(1-\varphi {)}^{n-1}$. ConsequentlyFor at least one hospital, $n>0$, this posterior is proper exactly when $s>0$. If every count is zero, the kernel diverges at $\varphi =0$ and does not define a posterior distribution; the improper prior cannot then be used for the subsequent expectation.