[R-sig-ME] Calculating R2 for a GLMM with small response values/Tweedie family

[Anthony R. Ives]

Skye,

I�d use a partial R2. I know that people like marginal and conditional R2�s, but I don�t understand how they can be interpreted as �amount of variation explained by�� A partial R2 explicitly asks how much loss-of-fit occurs with removal of a component (fixed or random) of a model. A (the) main problem solved by mixed models is violation of the exogeneity assumption of OLS, and marginal/conditional R2�s are insensitive to this problem.


You can calculate a partial R2 from likelihoods (from models fit with ML): see equation 17 in Ives, A.R. (2019. Rs for Correlated Data: Phylogenetic Models, LMMs, and GLMMs. Systematic Biology, 68, 234-251). You could grab the code from the package rr2, but I don�t think it can be used directly because it won�t recognize the class for glmmTMB. If you need a hand, just shoot me a private email.



Cheers, Tony


From: R-sig-mixed-models <r-sig-mixed-models-bounces using r-project.org> on behalf of Skye Bruce via R-sig-mixed-models <r-sig-mixed-models using r-project.org>
Date: Saturday, April 15, 2023 at 4:13 PM
To: r-sig-mixed-models using r-project.org <r-sig-mixed-models using r-project.org>
Subject: Re: [R-sig-ME] Calculating R2 for a GLMM with small response values/Tweedie family
Greetings,

I am emailing to ask about calculating R2 for a GLMM using Tweedie family of distributions in the glmmTMB + performance packages in R. My response variable contains many zeroes and very small values, and performance is having trouble calculating my model's distribution-specific variance, making the resulting R2 unreliable. Is there a more reliable way to calculate R2 for this GLMM?

I have measured my response variable, monarch butterfly eggs and larvae per m2, on grazed lands that vary in (predictors) grazing management category, floral richness, milkweed density (stems/m2) over two years, with three visits per year to each site.

Data .csv file attached.

Input:
mod1.egglarv1m.twe <- glmmTMB(egglarva1m ~ category + florarich + submilk1m + visit + year + category:year + (1|site),
                              data = egglarv1m.scale,
                              family = tweedie)
performance::r2(mod1.egglarv1m.twe)

Output:
# R2 for Mixed Models

  Conditional R2: 1.000
     Marginal R2: 0.724
Warning messages:
1: mu of 0.0 is too close to zero, estimate of random effect variances may be unreliable.
2: Can't calculate model's distribution-specific variance. Results are not reliable.

Thank you for your assistance,

Skye Bruce

PhD Student
University of Wisconsin�Madison | Entomology

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