I’ve written a blog post on how to compute R^2 for linear mixed models in JAGS using the method of Nakagawa et al. (2013). The post is available at the following URL: http://samcarcagno.altervista.org/stat_notes/r2_lmm_jags/r_squared_lmm.html
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Nakagawa, Shinichi, and Holger Schielzeth. 2013. “A general and simple method for obtaining R2 from generalized linear mixed-effects models.” Methods in Ecology and Evolution 4 (2): 133–42. doi:10.1111/j.2041-210x.2012.00261.x