Bayesian multilevel model in practice - selection of package, specification and interpretation

Question

I am trying to fit a Bayesian multilevel model in R and have several questions. I found two packages (brms and rstanarm) and am able to perform the analysis with both of them, so the technical part is not the problem. But:

• How do I decide for one of the two packages? Do they calculate differently or are they basically the same? Is there a way to tell if one the two should be preferred? If so, how?
• I did not specify any priors yet because I didn’t want to do anything wrong. However, I expect a positive effect because the treatment I examine has shown positive effects in numerous studies before (though with slightly different material, hence I cannot be completely sure either, especially about the magnitude of the effect). How is the best way to specify priors in a way that translates to “I’m not so sure about the magnitude but I expect a positive effect”?
• Is it common to just interpret the single parameters of my final model (e.g., median and MPE) or should I include the Bayes Factor for my central effect as well?

I never used Bayesian statistics in my work before, but I see the benefits and would like to get a better understanding. If anyone has any advice, general of specifically regarding my questions above, it would be greatly appreciated.

Here is what I mainly used to work with the brms package: https://cran.r-project.org/web/packages/brms/vignettes/brms_multilevel.pdf

And here is the tutorial I used for rstanarm: https://cran.r-project.org/web/packages/psycho/vignettes/bayesian.html#mixed-models

Answer 1

I just wanted to give an update for anyone interested:

• I understood that both rstanarm and brms use Stan for the computation of the Bayesian models. Therefore, I assumed that the decision between these two packages is not crucial and I opted for brms, because I found it more intuitive and found enough resources to conduct and interpret my analyses. A comparison between the packages is also included in the source I listed above (https://cran.r-project.org/web/packages/brms/vignettes/brms_multilevel.pdf).

• Because there is only little research on the specific effect I investigated, I decided to discard the specification of priors and sticked to the default prior (an improper flat prior over the reals, see https://www.jstatsoft.org/article/view/v080i01) for my parameters.

• I decided to interpret the mean of the posterior distribution and the respective 95% credible interval of all included parameters (including control variables). For my central effect (the one referring to my hypothesis) I also computed the evidence ratio, which can be interpreted as a one-sided Bayes factor (see https://github.com/paul-buerkner/brms/issues/311).

I will now mark my question as answered. However, if anyone with more expertise detects a mistake or is willing to answer my original questions I would still be grateful for hints and corrections.