A final method of defining priors goes under the name of reference priors (Berger et al 2009). The proposal starts from the observation that we should minimize the subjectivity of the results of our statistical analysis, and hence that we should minimize the impact of the prior probability on the posterior. The idea of reference priors is exactly that it will allow the sample data a maximal say in the posterior distribution. But since at the outset we do not know what sample we will obtain, the prior is chosen so as to maximize the expected impact of the data. The expectation must itself be taken with respect to some distribution over sample space, but again, it may well be that we have strong reasons for this latter distribution.