Compute Goodness-of-fit measures for various regression models, including mixed and Bayesian regression models.

cod(x)

r2(x, ...)

# S3 method for lme
r2(x, n = NULL, ...)

# S3 method for stanreg
r2(x, loo = FALSE, ...)

# S3 method for brmsfit
r2(x, loo = FALSE, ...)

## Arguments

x Fitted model of class lm, glm, merMod, glmmTMB, lme, plm, stanreg or brmsfit. For method cod(), only a glm with binrary response. Currently not used. Optional, an lme object, representing the fitted null-model (unconditional model) to x. If n is given, the pseudo-r-squared for random intercept and random slope variances are computed (Kwok et al. 2008) as well as the Omega squared value (Xu 2003). See 'Examples' and 'Details'. Logical, if TRUE and x is a stanreg or brmsfit object, a LOO-adjusted r-squared is calculated. Else, a rather "unadjusted" r-squared will be returned by calling rstantools::bayes_R2().

## Value

For r2(), depending on the model, returns:

• For linear models, the r-squared and adjusted r-squared values.

• For mixed models, the marginal and conditional r-squared values.

• For glm objects, Cox & Snell's and Nagelkerke's pseudo r-squared values.

• For brmsfit or stanreg objects, the Bayesian version of r-squared is computed, calling rstantools::bayes_R2().

• If loo = TRUE, for brmsfit or stanreg objects a LOO-adjusted version of r-squared is returned.

• Models that are not currently supported return NULL.

For cod(), returns the D Coefficient of Discrimination, also known as Tjur's R-squared value.

## Details

For linear models, the r-squared and adjusted r-squared value is returned, as provided by the summary-function.

For mixed models (from lme4 or glmmTMB) marginal and conditional r-squared values are calculated, based on Nakagawa et al. 2017. The distributional variance (or observation-level variance) is based on lognormal approximation, log(1+var(x)/mu^2).

For lme-models, an r-squared approximation by computing the correlation between the fitted and observed values, as suggested by Byrnes (2008), is returned as well as a simplified version of the Omega-squared value (1 - (residual variance / response variance), Xu (2003), Nakagawa, Schielzeth 2013), unless n is specified.

If n is given, for lme-models pseudo r-squared measures based on the variances of random intercept (tau 00, between-group-variance) and random slope (tau 11, random-slope-variance), as well as the r-squared statistics as proposed by Snijders and Bosker 2012 and the Omega-squared value (1 - (residual variance full model / residual variance null model)) as suggested by Xu (2003) are returned.

For generalized linear models, Cox & Snell's and Nagelkerke's pseudo r-squared values are returned.

The ("unadjusted") r-squared value and its standard error for brmsfit or stanreg objects are robust measures, i.e. the median is used to compute r-squared, and the median absolute deviation as the measure of variability. If loo = TRUE, a LOO-adjusted r-squared is calculated, which comes conceptionally closer to an adjusted r-squared measure.

## Note

cod()

This method calculates the Coefficient of Discrimination D for generalized linear (mixed) models for binary data. It is an alternative to other Pseudo-R-squared values like Nakelkerke's R2 or Cox-Snell R2. The Coefficient of Discrimination D can be read like any other (Pseudo-)R-squared value.

r2()

For mixed models, the marginal r-squared considers only the variance of the fixed effects, while the conditional r-squared takes both the fixed and random effects into account.

For lme-objects, if n is given, the Pseudo-R2 statistic is the proportion of explained variance in the random effect after adding co-variates or predictors to the model, or in short: the proportion of the explained variance in the random effect of the full (conditional) model x compared to the null (unconditional) model n.

The Omega-squared statistics, if n is given, is 1 - the proportion of the residual variance of the full model compared to the null model's residual variance, or in short: the the proportion of the residual variation explained by the covariates.

Alternative ways to assess the "goodness-of-fit" is to compare the ICC of the null model with the ICC of the full model (see icc).

## References

• DRAFT r-sig-mixed-models FAQ

• Bolker B et al. (2017): GLMM FAQ

• Byrnes, J. 2008. Re: Coefficient of determination (R^2) when using lme() (https://stat.ethz.ch/pipermail/r-sig-mixed-models/2008q2/000713.html)

• Kwok OM, Underhill AT, Berry JW, Luo W, Elliott TR, Yoon M. 2008. Analyzing Longitudinal Data with Multilevel Models: An Example with Individuals Living with Lower Extremity Intra-Articular Fractures. Rehabilitation Psychology 53(3): 370-86. doi: 10.1037/a0012765

• Nakagawa S, Schielzeth H. 2013. A general and simple method for obtaining R2 from generalized linear mixed-effects models. Methods in Ecology and Evolution, 4(2):133-142. doi: 10.1111/j.2041-210x.2012.00261.x

• Nakagawa S, Johnson P, Schielzeth H (2017) The coefficient of determination R2 and intra-class correlation coefficient from generalized linear mixed-effects models revisted and expanded. J. R. Soc. Interface 14. doi: 10.1098/rsif.2017.0213

• Rabe-Hesketh S, Skrondal A. 2012. Multilevel and longitudinal modeling using Stata. 3rd ed. College Station, Tex: Stata Press Publication

• Raudenbush SW, Bryk AS. 2002. Hierarchical linear models: applications and data analysis methods. 2nd ed. Thousand Oaks: Sage Publications

• Snijders TAB, Bosker RJ. 2012. Multilevel analysis: an introduction to basic and advanced multilevel modeling. 2nd ed. Los Angeles: Sage

• Xu, R. 2003. Measuring explained variation in linear mixed effects models. Statist. Med. 22:3527-3541. doi: 10.1002/sim.1572

• Tjur T. 2009. Coefficients of determination in logistic regression models - a new proposal: The coefficient of discrimination. The American Statistician, 63(4): 366-372

## Examples

data(efc)

# Tjur's R-squared value
efc$services <- ifelse(efc$tot_sc_e > 0, 1, 0)
fit <- glm(services ~ neg_c_7 + c161sex + e42dep,
data = efc, family = binomial(link = "logit"))
cod(fit)#>
#> R-Squared for (Generalized) Linear (Mixed) Model
#>
#> Tjur's D: 0.023
#>
library(lme4)#> Loading required package: Matrixfit <- lmer(Reaction ~ Days + (Days | Subject), sleepstudy)
r2(fit)#>
#> R-Squared for (Generalized) Linear (Mixed) Model
#>
#> Family : gaussian (identity)
#> Formula: ~Days | Subject Reaction ~ Days NA
#>
#>    Marginal R2: 0.279
#> Conditional R2: 0.799
#>
fit <- lm(barthtot ~ c160age + c12hour, data = efc)
r2(fit)#>
#> R-Squared for (Generalized) Linear (Mixed) Model
#>
#>          R-squared: 0.256