Past exam of the mathematical and Computational Biology course of the University of Cambridge / 2025 / Paper 1 / 6 / e

The two colonies with no observed cheaters are right-censored after seven days. Each contributes $\mathrm{Pr}(X>7)=(1-p{)}^7$, not the probability of an event on day seven or eight. The four observed first events contribute four factors of $p$ and $(6-1)+(7-1)+(5-1)+(5-1)=19$ failures. Including the two censored exposures givesThe beta distribution prior therefore yieldsIts probability density function is proportional to $p^8(1-p{)}^{37}$ for $0<p<1$: it vanishes at both endpoints, is unimodal, and has its mode at $8/45\simeq 0.17778$. The right panel of the sketch in question 5(b) shows this moderately right-skewed density. The calculation assumes daily observations detect the first event and that stopping the experiment after seven days is independent censoring.