Moving beyond noninformative priors: why and how to choose weakly informative priors in Bayesian analyses

21 March 2019

Lemoine, Nathan

Throughout the last two decades, Bayesian statistical methods have proliferated throughout ecology and evolution. Numerous previous references established both philosophical and computational guidelines for implementing Bayesian methods. However, protocols for incorporating prior information, the defining characteristic of Bayesian philosophy, are nearly nonexistent in the ecological literature. Here, I hope to encourage the use of weakly informative priors in ecology and evolution by providing a ``consumer's guide'' to weakly informative priors. The first section outlines three reasons why ecologists should abandon noninformative priors: (1) common flat priors are not always noninformative, (2) noninformative priors provide the same result as simpler frequentist methods, and (3) noninformative priors suffer from the same high Type I and Type M error rates as frequentist methods. The second section provides a guide for implementing informative priors, wherein I detail convenient "reference" prior distributions for common statistical models (i.e. regression, ANOVA, hierarchical models). I then use simulations to visually demonstrate how informative priors influence posterior parameter estimates. With the guidelines provided here, I hope to encourage the use of weakly informative priors for Bayesian analyses in ecology. Ecologists can and should debate the appropriate form of prior information, but should consider weakly informative priors as the new "default" prior for any Bayesian model.