I am currently writing up a data analysis plan for the preregistration. We conduct a pilot intervention study and compare two groups. For both groups we have baseline data and follow-up. Groups are randomized and should not differ in baseline scores. To assess the treatment effect, I want to perform an ANCOVA analysis and calculate the Bayes factor. It seems straightforward in JASP but I still have some questions.
From what I understood, we should first define the prior distribution which could be a half-normal distribution in our case, as smaller effects are more likely than larger effects. By default, the prior is defined as r scale fixed effect =0.5, r scale random effect =1, and r scale covariate = 0.354. What does this exactly mean and how can I change it to a half-normal distribution?
Similarly, there are two options: compare to best model or null model. Which one should be preferred? And should the covariate be added to the null model?
Thanks for helping out!