# Bayes factors and ROC curves

The question comes from Kevin Murphy's book, Ch 5, Ex 5.6. Could somebody suggest a solution?

Let $B=p(D|H_1)/p(D|H_0)$ be the bayes factor in favor of model 1. Suppose we plot two ROC curves, one computed by thresholding $B$, and the other computed by thresholding $p(H_1|D)$. Will they be the same or different? Why?

$B = \frac{p(D|H_1)}{p(D|H_0)} = \frac{p(H_1|D)p(H_0)}{p(H_0|D)p(H_1)} \propto \frac{p(H_1|D)}{p(H_0|D)}$
From this we see that ROC curve obtained by thresholding $B$ will be the same as ROC curve obtained by thresholding $p(H_1|D)$ only if $p(H_0|D)$ is constant.
• You can write the final term as $B \propto p/(1-p)$, so it’s just a monotonic transformation, leaving ROC invariant (assuming only 2 hypotheses) – innisfree May 13 '19 at 13:38
Sinc we are discussing ROC curves, let me assume we are taking about two hypothesis. We can write the Bayes factor as $$B_{10} = \frac{p_1}{1−p_1} \frac{\pi_0}{\pi_1}$$ where $$p_1$$ is the posterior of the alternative and $$\pi_0/\pi_1$$ are the prior odds.
Note well that the relationship between $$p_1$$ and $$B$$ is monotonic. The ROC is invariant under such monotonic transformations (as it only depends on the ordering of the discriminant, not it's absolute value). So the ROC curves will be the same.