Paper 4, Section II, H

Statistics
Part IB, 2018

There is widespread agreement amongst the managers of the Reliable Motor Company that the number XX of faulty cars produced in a month has a binomial distribution

P(X=s)=(ns)ps(1p)ns(s=0,1,,n;0p1)P(X=s)=\left(\begin{array}{c} n \\ s \end{array}\right) p^{s}(1-p)^{n-s} \quad(s=0,1, \ldots, n ; \quad 0 \leqslant p \leqslant 1)

where nn is the total number of cars produced in a month. There is, however, some dispute about the parameter pp. The general manager has a prior distribution for pp which is uniform, while the more pessimistic production manager has a prior distribution with density 2p2 p, both on the interval [0,1][0,1].

In a particular month, ss faulty cars are produced. Show that if the general manager's loss function is (p^p)2(\hat{p}-p)^{2}, where p^\hat{p} is her estimate and pp the true value, then her best estimate of pp is

p^=s+1n+2\hat{p}=\frac{s+1}{n+2}

The production manager has responsibilities different from those of the general manager, and a different loss function given by (1p)(p^p)2(1-p)(\hat{p}-p)^{2}. Find his best estimate of pp and show that it is greater than that of the general manager unless s12ns \geqslant \frac{1}{2} n.

[You may use the fact that for non-negative integers α,β\alpha, \beta,

01pα(1p)βdp=α!β!(α+β+1)!]\left.\int_{0}^{1} p^{\alpha}(1-p)^{\beta} d p=\frac{\alpha ! \beta !}{(\alpha+\beta+1) !}\right]