Past exam of the mathematics course of the University of Cambridge / 2017 / ii / Paper 1 / 28K / a / Solution

For observation $X=k$, the likelihood is proportional to $p^k(1-p{)}^{n-k}$. For $0<k<n$, its logarithmic derivative is $k/p-(n-k)/(1-p)$, vanishing uniquely at $k/n$; the logarithm is strictly concave. At $k=0$ or $k=n$, monotonicity gives the boundary maximum. Thus the maximum-likelihood estimator isA uniform prior multiplies this likelihood by a constant. The beta-integral normalization therefore gives the posterior distributionIn particular the posterior mean $(k+1)/(n+2)$ is not automatically the optimal estimator for the weighted loss in the next part.