Methods & Meta-science
How to obtain evidence for the null hypothesis
To get evidence for or against one's theory relative to the null hypothesis, one needs to know what it predicts. The amount of evidence can then be quantified by a Bayes factor. It is only when one has reasons for specifying a scale of effect that the level of evidence can be specified for no effect (that is, non-significance is not a reason for saying there is no effect). In almost all papers I read people declare absence of an effect while having no rational grounds for doing so. So we need to specify what scale of effect our theory predicts. Specifying what one's theory predicts may not come naturally, but I show some ways of thinking about the problem. I think our science will be better for it!