R^2 for linear mixed effects models

Linear mixed effects models are a powerful technique for the analysis of ecological data, especially in the presence of nested or hierarchical variables. But unlike their purely fixed-effects cousins, they lack an obvious criterion to assess model fit.

[Updated October 13, 2015: Development of the R function has moved to my piecewiseSEM package, which can be found here under the function sem.model.fits]

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