# P-values and Posterior Probabilities

10 Apr 2017Alternative click-bait style title:

## Given a p-value of 0.045 what is the probability that the null model is true? The answer will surprise you!

You might want to argue that the question is based on a false premise; the p-value is related to the probability of falsely rejecting the null hypothesis conditional on the level () and the null hypothesis being true. The question of whether the null-model valid or not is only assesed from this viewpoint and we certainly cannot assign a probability it.

However in Bayesian statistics such a question is perfectly valid. The theory of Bayesian robustness examines how sensitive a Bayesian inference is to the choice of prior. Thereby adressing one of the main criticisms of Bayesian statistics. In a seminal paper James O. Berger and Thomas Sellke consider a single realization (the results still holds for multiple realizations) of a normal variable with known variance: and a test for the hypotesis against the alternative Letâ€™s also phrase a Bayesian model; we give each of the two models an equal prior probability We also need a prior for under , . Berger and Sellke shows that if corresponding to a p-value less than , the prior favoring the most within the class of symmetric unimodal distributions is a uniform distribution,

We find the that maximizes the posterior probability of the alternative for different values of and their corresponding posterior probabilities and p-values.

This answers our question: With the prior that *favors the alternative the most* the posterior probability that the null hypothesis is true for a p-value of 0.045 is still . Not exactly overwhelming evidence for the alternative.

Below we have plotted the posterior probability for against the p-value for the prior that favors the alternative the most.