Evidence-appraisal glossary
Bayes Factor
A number that weighs how well the observed data fit one hypothesis versus another, showing which the evidence favors and by how much. Unlike a p-value, it can support the null hypothesis as well as the alternative.
Also called: Bayes factor.
The Bayes factor is the ratio of the probability of the data under one hypothesis to their probability under a competing one, turning evidence into a direct comparison. A value well above one favors the first hypothesis, well below one favors the second, and near one means the data barely distinguish them. This lets it express something a p-value cannot: genuine support for no effect, rather than mere failure to reject. Its value depends on the prior assumptions chosen for each hypothesis, so those choices deserve scrutiny.
Read the full Reading the Evidence blog.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.