drmTMB

Model more than the average

drmTMB is an R package that fits distributional regression models (DRMs) for one or two responses. Distributional regression lets predictors change not only an outcome's average, but also other features of its distribution: for example, how variable it is or how likely it is to be zero.

Use it for questions such as: does temperature change average growth; is growth less predictable in one habitat; or, after accounting for their averages and variability, do two responses still vary together? The worked examples come from ecology, evolution, and environmental science.

drmTMB 0.7.1 is the development version; 0.7.0 is on CRAN. The package is experimental: start with a documented example, check the fitted model, and read Can I fit and report this model? before reporting a result.

A first model

A Gaussian location-scale model gives the response two formulas: one for its location mu (here, the expected response) and one for its scale sigma (here, the residual standard deviation, SD). In this example, habitat and temperature change mean growth, and habitat also changes the residual SD:

growth_i ~ Normal(mu_i, sigma_i^2)
mu_i         = beta_0  + beta_1 grassland_i + beta_2 temperature_i
log(sigma_i) = gamma_0 + gamma_1 grassland_i

i indexes observations, grassland_i is 1 for grassland and 0 for forest, the beta coefficients belong to the mu formula, and the gamma coefficients belong to the sigma formula. The simulation below sets beta = (1, 0.6, 0.4) and gamma = (-0.5, 0.45):

library(drmTMB)

set.seed(1)
n <- 120
dat <- data.frame(
  habitat = factor(rep(c("forest", "grassland"), each = n / 2)),
  temperature = rnorm(n)
)
mu <- 1 + 0.6 * (dat$habitat == "grassland") + 0.4 * dat$temperature
sigma <- exp(-0.5 + 0.45 * (dat$habitat == "grassland"))
dat$growth <- rnorm(n, mean = mu, sd = sigma)

fit <- drmTMB(
  drm_formula(growth ~ habitat + temperature, sigma ~ habitat),
  family = gaussian(),
  data = dat
)

check_drm(fit)
#> <drm_check: 16 checks>
#> ok: 16; notes: 0; warnings: 0; errors: 0
#> ... (one row per check, omitted here)

summary(fit)
#> <summary.drmTMB>
#> estimator: ML
#>                          estimate  std_error
#> mu:(Intercept)          0.9703459 0.07991617
#> mu:habitatgrassland     0.5467444 0.15077857
#> mu:temperature          0.4134070 0.07810551
#> sigma:(Intercept)      -0.4851669 0.09128765
#> sigma:habitatgrassland  0.4776546 0.12910105
#> ... (fitted-sigma range, logLik, and convergence code omitted here)

check_drm() checks optimizer convergence, gradients, the Hessian, standard errors, dropped rows, and fitted scales. A clean fit reports zero notes, warnings, and errors, as above; resolve any warning or error before reading estimates. In summary(fit), mu: rows are the beta coefficients on the response scale and sigma: rows are the gamma coefficients on the log-SD scale. Each estimate here is within one standard error of its simulated value. A positive sigma:habitatgrassland means grassland growth is more variable than forest growth; exponentiate it to report an SD ratio:

sigma_habitat <- coef(fit, "sigma")["habitatgrassland"]
exp(sigma_habitat)      # grassland / forest residual SD ratio: 1.61
exp(2 * sigma_habitat)  # residual variance ratio: 2.60
head(sigma(fit))        # fitted residual SD for each observation

The getting-started article fits the same example and walks through the full summary() output.

Parameter names

Each distributional parameter has one formula name and one plain-English name, used throughout the documentation:

Formula name	Name in the guides	What it describes
mu (mu1, mu2)	location	Where the response distribution sits; for a Gaussian response, the expected response.
sigma (sigma1, sigma2)	scale	Spread around mu; for a Gaussian response, the residual SD, modelled on the log scale.
nu and other family-specific names	shape	Features beyond location and scale, such as Student-t tail weight.
zi, hu	zero inflation, hurdle	The probability of an extra zero, or of a zero from a separate hurdle process.
rho12	residual correlation (coscale)	How two responses vary together within an observation after their mu and sigma models.

If you already use lme4 or glmmTMB

The mu formula is an ordinary lme4/glmmTMB formula, random-effect terms such as (1 | site) included. Each further parameter gets its own named formula; for a Gaussian model, the sigma formula plays the role of glmmTMB's dispformula:

glmmTMB::glmmTMB(y ~ x + (1 | site), dispformula = ~ x, data = dat)
drmTMB(bf(y ~ x + (1 | site), sigma ~ x), family = gaussian(), data = dat)

bf() is a short alias for drm_formula(); its name and layout follow brms::bf(). Comparing drmTMB with other packages fits the same models side by side in drmTMB and in lm(), lme4, glmmTMB, metafor, and other packages.

Install

Install the released version from CRAN:

install.packages("drmTMB")

To try development changes before a release, install from GitHub with pak:

install.packages("pak")
pak::pak("itchyshin/drmTMB")

You need R 4.1.0 or newer and a working compiler toolchain because TMB models are compiled during installation. If installation fails while compiling C++, install the usual R build tools for your platform: Rtools on Windows, Xcode Command Line Tools on macOS, or the R development toolchain on Linux.

The installer also installs the packages drmTMB imports (cli, lifecycle, Matrix, nlme, and TMB, plus base R's methods, stats, and utils) and the compile-time headers from RcppEigen and TMB. The articles also use suggested packages, including glmmTMB, lme4, MASS, metafor, knitr, and rmarkdown; the Suggests field of DESCRIPTION lists them all.

The GitHub install does not build the vignettes, so vignette("drmTMB") reports that the vignette is not found. Read the articles on the package website, install the CRAN release, or build them locally with remotes::install_github("itchyshin/drmTMB", build_vignettes = TRUE), which also needs the suggested packages.

Start with your scientific question

Choose one route. Each begins with a runnable example and tells you what to check before interpreting the result.

If your data and question are...	Start here	Important limit
One response, and predictors may change its average or residual variability	Distributional regression with drmTMB	Begin with the simplest response family that matches the data, and run check_drm() after every fit.
A trait measured across related species, with a tree	Phylogenetic mixed models	The first worked route is Gaussian. It needs repeated observations within species to separate phylogenetic from residual variation.
One Gaussian response measured at named sites with coordinates	Coordinate-spatial structured effects	Start with a spatial mu intercept. Repeated observations within sites are needed to separate spatial from residual variation; this route does not establish range estimates or intervals.
Effect sizes with known sampling variances or covariance	Mean effects and residual heterogeneity	This is Gaussian regression with meta_V(V = V), not a response family for raw observations.

For a count, proportion, zero-heavy, robust, bivariate, pedigree, or known-matrix analysis, use What can I fit today? to find the relevant guide.

What can I fit today?

The table summarises what is fitted in 0.7.1. The model map gives the exact forms and the guide for each row, and Can I fit and report this model? says which estimates and intervals are ready to report.

Model	Fitted now	Not yet supported
One response, fixed effects	One formula per family parameter for Gaussian, Student-t, skew-normal, lognormal, Gamma, Tweedie, beta, zero-one beta, beta-binomial, binomial, Poisson, NB2, zero-inflated Poisson and NB2, truncated NB2, hurdle NB2, and cumulative-logit ordinal responses; ordinal disc ~ z fixed-effect discrimination	Ordinal random-effect discrimination and cutpoint-specific predictors, ordered beta, predictor-dependent Tweedie power
Gaussian random effects	mu and sigma random intercepts, independent slopes, and correlated intercept-slope blocks	Covariance between mu and sigma slopes across formulas
Non-Gaussian random effects	mu random intercepts and independent numeric slopes for Student-t, skew-normal, lognormal, Gamma, Tweedie, beta, zero-one beta, beta-binomial, binomial, Poisson, NB2, truncated NB2, and cumulative-logit responses	Correlated non-Gaussian slopes
Random-effect SD models	sd(group) ~ x_group for Gaussian mu random intercepts	Slope-specific sd() formulas
Meta-analysis	Gaussian meta_V(V = V) with diagonal, dense, or bivariate known sampling covariance	Non-Gaussian or sparse known covariance
Two responses	Bivariate Gaussian mu1, mu2, sigma1, sigma2, and predictor-dependent rho12; matching random intercepts across the two responses	Random effects in rho12, mixed-family pairs
Phylogenetic, spatial, pedigree, and known-matrix effects	Gaussian mu and sigma intercepts and one numeric mu slope; Poisson and NB2 mu intercept plus one slope	Multiple or labelled structured slopes, structured rho12, simultaneous phylogenetic and spatial terms
Temporal AR1, OU, and Toeplitz effects	One univariate Gaussian ML route: temporal(1 | id, time = occasion, structure = "ar1"), temporal(1 | id, time = elapsed, structure = "ou"), or, for a complete equally spaced panel of 3--12 occasions, temporal(1 | id, time = occasion, structure = "homtoep") in the formula. AR1 mean-coefficient Wald intervals can be computed when the full Hessian permits; fixed-mean profiles are computable on named AR1, OU, and Toeplitz routes. OU and Toeplitz profile coverage evidence is confined to the article's exact cells; AR1 has no calibrated coverage claim. See

Current boundaries

drmTMB fits one-response and two-response models only.

A model that fits is not proof that its uncertainty is reliable. Do not assume that a family example extends to multiple random effects, a new structured effect, missing data, or a different interval method. Check Can I fit and report this model? for those cases, and inspect check_drm() and the interval output before reporting a complex model.

The residual correlation rho12 describes how two responses vary together within an observation. It is different from a correlation among groups, species, or locations, which drmTMB reports as separate covariance summaries. Use the model map when your question involves more than one kind of correlation.

Maximum likelihood and REML

drmTMB fits by maximum likelihood (ML) unless you set REML = TRUE. Use ML to compare models with different fixed effects. Restricted maximum likelihood (REML), the lme4::lmer() default, is for estimating variance components in the documented Gaussian models, after the fixed effects are chosen. Model selection with AIC and BIC shows both steps.

Engines

drmTMB uses Template Model Builder (TMB) to fit models by default. DRModels.jl is an optional Julia companion. For supported models, the Julia engine guide shows how to select it from R with engine = "julia" and explains its limits. engine = "julia" currently admits Gaussian, bivariate Gaussian, Student-t, lognormal, Poisson, NB2, Gamma, Beta, and binomial models on the fixed-effect route (several also have a large-p phylo() or coupled location-scale phylo route), plus truncated_nbinom2(), zero_one_beta(), tweedie(), beta_binomial(), skew_normal(), cumulative_logit(), biv_student(), and biv_lognormal() on the fixed-effect route only. Julia is not required to install or use the default R workflow.

Project status

The package is under active development. See Can I fit and report this model?, the reference index, and the articles above for the current fitted workflows.