Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2008/iii/paper-44/1/h/solution

For , both event probabilities tend to one, so the posterior odds tend to . If , the event's Bayes factor decreases toward one. To see monotonicity, put and write
The last quotient is a weighted average of the decreasing values , with weights . Adding the next, smaller value decreases the average.
Thus the selection rule becomes less informative as more teams try: eventually a positive study is almost certain even under the null. This explains the hotter-field interpretation in Ioannidis's model. It describes evidence retained by this particular selection event, rather than proving that every individual study becomes less reliable or that the full accumulated evidence gets weaker.

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