-Over the years, many \proglang{R} packages have been developed that implement MLMs, each being more or less general in their supported models. Comparing all of them to \pkg{brms} would be too extensive and barely helpful for the purpose of the present paper. Accordingly, we concentrate on a comparison with four packages. These are \pkg{lme4} \citep{bates2015} and \pkg{MCMCglmm} \citep{hadfield2010}, which are possibly the most general and widely applied \proglang{R} packages for MLMs, as well as \pkg{rstanarm} \citep{rstanarm2016} and \pkg{rethinking} \citep{mcelreath2016}, which are both based on \pkg{Stan}. As opposed to the other packages, \pkg{rethinking} was primarily written for teaching purposes and requires the user to specify the full model explicitly using its own simplified \pkg{BUGS}-like syntax thus helping users to better understand the models that are fitted to their data. However, this may be a rather time-consuming (and sometimes hard to debug) process as compared to the classical \proglang{R} formula syntax used in the other packages.
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