This page is a reference for checking functions and methods while analyzing data with BayesRTMB.

If you want the overall workflow first, see Quick Start. If you want to write custom models, see Writing Model Codes. If you want the details of the inference algorithms, see RTMB Internals and Inference Algorithms. This page collects the items that you often need to check during an analysis.

0. How To Read This Page

Most code chunks on this page use eval = FALSE. Some examples are complete examples that can be run as-is, while others are partial code fragments intended to be placed inside rtmb_code(). When only setup = { ... }, model = { ... }, or generate = { ... } is shown, read it as the code for that block.

The name fit is used as a placeholder for any fit object. In real analyses, replace it with objects such as fit_mcmc, fit_map, fit_vb, or fit_classic, depending on the estimation method.

The following terms are used throughout this page.

Term	Meaning
fit object	Estimation result returned by `sample()`, `optimize()`, `variational()`, or `classic()`
primary parameters	Estimated parameters declared in the `parameters` block
transform	Derived quantities created in the `transform` block
generate	Post-estimation derived quantities created in the `generate` block
draw	Sample from a posterior or approximate posterior distribution

Goal-Oriented Quick Navigation

Goal	Start here	Functions and methods
Run ordinary Bayesian inference	Section 4 MCMC_Fit	`mdl$sample()`, `fit$summary()`, `fit$diagnose()`
Create MAP estimates or MCMC initial values	Section 5 MAP_Fit	`mdl$optimize(num_estimate = ...)`, `fit$estimate()`
Explore a model quickly with VB	Section 6 VB_Fit	`mdl$variational()`, `fit$plot_elbo()`, `fit$diagnose()`
Use classic AIC/BIC/ANOVA	Section 7 Classic_Fit	`mdl$classic()`, `AIC()`, `BIC()`, `anova()`
Compare models	Section 8 Model comparison	`fit$bayes_factor()`, `fit$WAIC()`, `AIC()`, `BIC()`
Fix parameters	Section 9 fixed	`fixed = list(...)`, `mdl$fixed_model()`
Write a custom model	Sections 10-14	`rtmb_code()`, `Dim()`, distributions, AD-taping notes
Use an old fit object with current methods	Section 15.9	`upgrade_fit()`

1. Analysis Workflow

A BayesRTMB analysis usually follows this path.

wrapper function or rtmb_code()
  -> RTMB_Model
    -> sample()       -> MCMC_Fit
    -> optimize()     -> MAP_Fit
    -> variational()  -> VB_Fit
    -> classic()      -> Classic_Fit

When you use a wrapper function, functions such as rtmb_lm() and rtmb_glmer() return an RTMB_Model directly.

library(BayesRTMB)

mdl <- rtmb_lm(
  sat ~ talk * perf,
  data = debate,
  prior = prior_normal()
)

fit_mcmc <- mdl$sample()
fit_map  <- mdl$optimize()
fit_vb   <- mdl$variational()

When writing a custom model, define the model with rtmb_code() and pass it to rtmb_model(). For new users, it is often clearest to pass a data frame as data and connect data-frame columns to model variables in setup.

df <- debate

code <- rtmb_code(
  setup = {
    Y <- sat
    X <- talk

    center <- function(x) x - mean(x)
    X_c <- center(X)
  },
  parameters = {
    Intercept <- Dim()
    b <- Dim()
    sigma <- Dim(lower = 0)
  },
  transform = {
    mu <- Intercept + b * X_c
  },
  model = {
    Y ~ normal(mu, sigma)
    Intercept ~ normal(0, 10)
    b ~ normal(0, 10)
    sigma ~ exponential(1)
  }
)

mdl <- rtmb_model(
  data = df,
  code = code
)

In this example, data = df makes df$sat and df$talk available inside setup. The setup block is the place for preprocessing that should be decided once before model evaluation, such as handling missing data, creating indexes, building design matrices, and defining helper functions.

After estimation, use methods such as summary(), estimate(), diagnose(), and draws() on the fit object.

fit_mcmc$summary()
fit_mcmc$diagnose()
fit_mcmc$EAP(pars = "parameters")
fit_mcmc$MAP(pars = "parameters")

1.1 Choosing An Estimation Method

When in doubt, start with this table.

Goal	Recommendation
Final Bayesian inference	`sample()`
Check ESS, R-hat, and divergences	`MCMC_Fit$diagnose()`
Compute a Bayes factor	`MCMC_Fit$bayes_factor()`
Compute WAIC	`WAIC = TRUE` and `fit$WAIC()`
Fast point estimation	`optimize()`
Initial values for MCMC	`optimize(num_estimate = ...)`
Explore a large model	`variational()`
Check VB convergence	`VB_Fit$plot_elbo()`
Use AIC/BIC/ANOVA	`classic()`
Use robust standard errors	`Classic_Fit$robust_se()`
Fix parameters	`fixed = list(...)` or `$fixed_model()`
Rotate MDU or factor-analysis axes	`$rotate()` or `$fa_rotate()`

2. Function Overview

2.1 Model Definition

Function	Purpose
`rtmb_code()`	Define a model with `setup`, `parameters`, `transform`, `model`, and `generate` blocks
`rtmb_model()`	Create an `RTMB_Model` from `rtmb_code()` and data
`Dim()`	Declare parameter dimensions, constraints, types, and `random` status
`print_code()`	Inspect model code generated by a wrapper
`upgrade_fit()`	Rebuild old saved fit objects with the current class definitions

2.2 Wrapper Functions

Function	Main use
`rtmb_lm()`	Normal linear regression
`rtmb_glm()`	Generalized linear models
`rtmb_lmer()`	Linear mixed models
`rtmb_glmer()`	Generalized linear mixed models
`rtmb_ttest()`	Bayesian/classic t tests, effect sizes, Bayes factors
`rtmb_corr()`	Correlations, partial correlations, correlation matrices
`rtmb_fa()`	Factor analysis
`rtmb_irt()`	Item response theory models
`rtmb_lrt()`	Latent rank theory
`rtmb_mdu()`	Multidimensional unfolding
`rtmb_mediation()`	Mediation analysis
`rtmb_mixture()`	Mixture distribution models
`rtmb_table()`	Contingency-table models
`rtmb_loglinear()`	Log-linear models

2.3 Estimation Methods

Method	Return value	Main use
`$sample()`	`MCMC_Fit`	Final Bayesian inference, intervals, Bayes factors, WAIC
`$optimize()`	`MAP_Fit`	MAP/ML/marginal MAP, fast point estimates, initial-value search
`$variational()`	`VB_Fit`	Approximate posterior inference and model exploration
`$classic()`	`Classic_Fit`	Frequentist estimation for flat-prior wrapper models, AIC/BIC, ANOVA

sample() is the standard route for Bayesian inference. optimize() and variational() are useful for quick checks and initial-value search, but MCMC is preferred for uncertainty evaluation and Bayes factors.

3. Common Fit-Object Methods

MCMC_Fit, MAP_Fit, VB_Fit, and Classic_Fit share several methods. The meaning and availability of each method still depends on the fit-object type.

In this section, fit means any fit object. Read it as fit_mcmc for MCMC, fit_map for MAP, fit_vb for VB, and fit_classic for classic estimation. In particular, EAP() and MAP() are mainly for MCMC/VB, while classic point estimates are usually obtained with estimate().

Method	Purpose
`$estimate()`	Extract estimates as a list or as a single object
`$EAP()`	Extract posterior means, mainly for MCMC/VB
`$MAP()`	Extract marginal or joint MAP estimates, mainly for MCMC/VB
`$summary()`	Tabular estimation results
`$print()`	Print `summary()`
`$diagnose()`	Diagnostics appropriate to the fit object
`$rotate()`	Procrustes rotation for matrix parameters such as MDU coordinates
`$fa_rotate()`	Factor-analysis rotation for loadings

3.1 `estimate()`, `EAP()`, and `MAP()`

estimate() is the common API for extracting parameters, transformed quantities, and generated quantities.

fit$estimate(pars = "parameters")
fit$estimate(pars = "transform")
fit$estimate(pars = "generate")
fit$estimate(pars = "all")

pars = "parameters" returns only the primary parameters declared in the parameters block. This is usually what you want when reusing estimates as initial values or fixed values.

est <- fit$estimate(pars = "parameters", drop = FALSE)

drop = TRUE is the default. When only one parameter is selected, the vector, matrix, or array itself is returned instead of a list. Use drop = FALSE if you always want a list.

b_eap <- fit$EAP("b")
b_eap_list <- fit$EAP("b", drop = FALSE)

EAP() is a shortcut for estimate(type = "EAP"). MAP() returns marginal MAP estimates by default.

fit$EAP(pars = "parameters")
fit$MAP(pars = "parameters")
fit$MAP(pars = "parameters", type = "joint")

3.2 `pars` And `component`

Use pars and component to narrow what is extracted.

fit$estimate(pars = c("b", "sigma"))
fit$estimate(component = "transform")
fit$estimate(component = "generate")
fit$estimate(pars = "-theta")

For MCMC/VB, pars = NULL returns primary parameters by default. For classic and MAP fits, results may also include estimated fixed effects, transformed quantities, and generated quantities.

4. MCMC_Fit

MCMC_Fit is returned by sample(). It is the central object for analyses that use the full posterior distribution, including Bayes factors, WAIC, posterior predictive checks, rotations, and generated quantities.

fit_mcmc <- mdl$sample(
  chains = 4,
  warmup = 1000,
  sampling = 1000,
  metric = "auto",
  nuts_variant = "multinomial"
)

4.1 Main Methods

Method	Purpose
`$draws()`	Extract posterior draws as a 3D array `[iteration, chain, variable]`
`$summary()`	Display mean, sd, MAP, quantiles, ESS, and R-hat
`$rhat_summary()`	Summarize R-hat values
`$diagnose()`	Diagnose acceptance, treedepth, divergence, ESS, R-hat, and metric behavior
`$EAP()`, `$MAP()`	Extract posterior mean or MAP estimates
`$transformed_draws()`	Compute transformed quantities for posterior draws
`$generated_quantities()`	Compute generated quantities for posterior draws
`$WAIC()`	Compute WAIC from pointwise `log_lik`
`$bayes_factor()`	Compute Bayes factors by bridge sampling
`$rotate()`	Rotate matrix-valued parameters
`$fa_rotate()`	Rotate factor loadings and related factor-analysis quantities

4.2 `draws()`

draws() extracts posterior draws. By default, it returns fixed parameters and available transformed/generated quantities.

fit_mcmc$draws()
fit_mcmc$draws(pars = "b")
fit_mcmc$draws(include_random = TRUE)
fit_mcmc$draws(include_transform = FALSE, include_generate = FALSE)

For large models, narrow pars before extracting draws to avoid unnecessary memory use.

4.3 `summary()` And `diagnose()`

summary() gives the estimation table. diagnose() summarizes sampling quality.

fit_mcmc$summary()
fit_mcmc$diagnose()
fit_mcmc$rhat_summary()

Pay special attention to:

R-hat values close to 1.
Effective sample size.
Divergences.
Maximum treedepth hits.
Metric adaptation and positive-definite fallback messages.

4.4 `transformed_draws()` And `generated_quantities()`

Use these methods when you want derived quantities for each posterior draw.

fit_mcmc$transformed_draws()
fit_mcmc$generated_quantities()

If a generate block contains log_lik, generated_quantities() is the basis for WAIC and related pointwise predictive criteria.

4.5 Bridge Sampling And Bayes Factors

Bayes factors compare two fitted models. In BayesRTMB, Bayes factors are usually computed from MCMC fits.

fit_full <- mdl_full$sample()
fit_null <- mdl_null$sample()

fit_full$bayes_factor(fit_null)

When comparing models, use the same data and compatible priors, and check MCMC diagnostics for both models. Bayes factors are sensitive to the prior on parameters that differ between models.

4.6 WAIC

To compute WAIC, the model must provide pointwise log-likelihood values. For wrappers, set WAIC = TRUE when creating the model.

mdl <- rtmb_lm(
  sat ~ talk * perf,
  data = debate,
  WAIC = TRUE
)

fit <- mdl$sample()
fit$WAIC()

For custom models, return or report log_lik in the generate block.

generate = {
  log_lik <- normal_lpdf(Y, mu, sigma, sum = FALSE)
  report(log_lik)
}

5. MAP_Fit

MAP_Fit is returned by optimize(). It is useful for fast point estimation, initial-value search, profiling, and constrained/fixed-parameter workflows.

fit_map <- mdl$optimize(num_estimate = 10)

5.1 Main Methods

Method	Purpose
`$estimate()`	Extract MAP estimates
`$summary()`	Display estimates and standard errors when available
`$diagnose()`	Summarize optimization results
`$profile()`	Profile selected parameters
`$WAIC()`	Compute WAIC when generated `log_lik` is available
`$rotate()`	Rotate matrix-valued parameters
`$fa_rotate()`	Rotate factor-analysis results

5.2 `num_estimate` And The Best Run

Optimization can be sensitive to initial values. Use multiple starts with num_estimate.

fit_map <- mdl$optimize(num_estimate = 20)
fit_map$diagnose()

BayesRTMB keeps optimization history and selects the best run according to the objective value and convergence status.

5.3 Using Optimization Results As Initial Or Fixed Values

MAP estimates can be reused as initial values for MCMC.

init <- fit_map$estimate(pars = "parameters", drop = FALSE)
fit_mcmc <- mdl$sample(init = init)

They can also be used to construct fixed models.

est <- fit_map$estimate(pars = "parameters", drop = FALSE)
mdl_fixed <- mdl$fixed_model(fixed = list(b = est$b))

5.4 `profile()`

profile() evaluates the objective around selected parameters. It is useful for checking local curvature, asymmetric uncertainty, and non-quadratic likelihood shape.

fit_map$profile(pars = "b")

5.5 WAIC For MAP Fits

WAIC is usually interpreted as a posterior predictive criterion and is most natural for MCMC. For MAP fits, WAIC is available when generated log_lik values can be computed, but it should be interpreted as an approximation.

6. VB_Fit

VB_Fit is returned by variational(). It approximates the posterior distribution and is useful for exploring larger models or preparing MCMC.

fit_vb <- mdl$variational(iter = 3000)

6.1 Main Methods

Method	Purpose
`$EAP()`	Extract variational posterior means
`$MAP()`	Extract approximate MAP-like summaries
`$summary()`	Display variational summaries
`$plot_elbo()`	Plot ELBO trajectory
`$diagnose()`	Diagnose convergence and stability
`$draws()`	Extract approximate posterior draws
`$WAIC()`	Compute WAIC when `log_lik` is available

6.2 `num_estimate` And Best Estimate

VB can run several independent estimates and select the best ELBO.

fit_vb <- mdl$variational(num_estimate = 10)
fit_vb$plot_elbo()

By default, EAP() and MAP() use the best estimate.

fit_vb$EAP()
fit_vb$EAP(chains = 1)
fit_vb$EAP(best_chains = 2)

6.3 Checking Convergence With `plot_elbo()`

Use plot_elbo() to check whether the ELBO has stabilized.

fit_vb$plot_elbo()
fit_vb$diagnose()

If the ELBO remains unstable, increase iter, try different initial values, or use VB only as an exploratory tool before MCMC.

6.4 WAIC For VB Fits

WAIC can be computed from VB draws when log_lik is available. Interpret it as an approximation, and prefer MCMC-based WAIC for final reporting when feasible.

7. Classic_Fit

Classic_Fit is returned by classic(). It provides frequentist-style output for supported wrapper models, especially flat-prior models.

mdl <- rtmb_lm(sat ~ talk * perf, data = debate)
fit_classic <- mdl$classic()
fit_classic$summary()

7.1 Main Methods

Method	Purpose
`$estimate()`	Extract point estimates
`$summary()`	Display estimates, standard errors, and tests
`$diagnose()`	Check fit diagnostics where available
`$robust_se()`	Compute robust standard errors
`$lsmeans()`	Compute least-squares means where supported
`AIC()`, `BIC()`	Information criteria
`anova()`	Model comparison or analysis-of-variance tables

7.2 ANOVA

Use anova() for supported classic fits.

fit1 <- rtmb_lm(sat ~ talk, data = debate)$classic()
fit2 <- rtmb_lm(sat ~ talk * perf, data = debate)$classic()

anova(fit1, fit2)

7.3 AIC, BIC, And `logLik`

AIC(fit_classic)
BIC(fit_classic)
logLik(fit_classic)

These criteria are most natural for classic/MAP likelihood-based fits. For Bayesian predictive comparison, use WAIC or posterior predictive checks.

7.4 `robust_se()`

robust_se() returns a copy of the fit with robust standard errors by default.

fit_robust <- fit_classic$robust_se(type = "HC3")
fit_cluster <- fit_classic$robust_se(cluster = debate$group)

To update the fit object itself, use the update option if supported.

fit_classic$robust_se(type = "HC3", update = TRUE)

7.5 `lsmeans()`

lsmeans() computes adjusted marginal means for supported models.

fit_classic$lsmeans(~ talk)
fit_classic$lsmeans(~ talk | perf)

8. Model Comparison And Evaluation Criteria

BayesRTMB supports several model comparison tools. Choose the criterion according to the estimation method and the question.

Criterion	Main fit type	Main interpretation
Bayes factor	MCMC	Evidence ratio between Bayesian models
WAIC	MCMC/VB/MAP with `log_lik`	Approximate out-of-sample predictive fit
AIC/BIC	Classic/MAP	Likelihood-based information criteria
ELBO	VB	Variational objective, useful for VB runs

8.1 Bayes Factor

Bayes factors are usually computed by bridge sampling from MCMC fits.

bf <- fit_full$bayes_factor(fit_null)
bf

Use Bayes factors when the prior is part of the scientific question. Do not compare models with incompatible data, likelihoods, or unintended prior changes.

8.2 WAIC

WAIC requires pointwise log-likelihood values.

mdl <- rtmb_glm(y ~ x, data = dat, family = "bernoulli", WAIC = TRUE)
fit <- mdl$sample()
fit$WAIC()

For custom models, create log_lik in generate.

8.3 AIC/BIC

Use AIC/BIC for likelihood-based classic or optimization fits.

AIC(fit_classic)
BIC(fit_classic)

8.4 ELBO

For VB, the ELBO is mainly a diagnostic and selection criterion among VB runs.

fit_vb$plot_elbo()
fit_vb$diagnose()

9. Fixing Parameters With `fixed`

fixed lets you fix one or more parameters to specified values. It can be used when creating a model or by calling $fixed_model().

mdl_fixed <- rtmb_lm(
  sat ~ talk * perf,
  data = debate,
  fixed = list(b = c(0, 0))
)

mdl_fixed2 <- mdl$fixed_model(
  fixed = list(sigma = 1)
)

Fixed values should match the parameter names and dimensions used internally by the model.

9.1 Checking Parameter Names

Use summaries, estimates, draws, and printed model code to check the names.

fit$summary()
fit$estimate(pars = "parameters", drop = FALSE)
fit_mcmc$draws(pars = "parameters")
mdl$print_code()

9.2 Internal Handling Of `fixed`

Internally, fixed parameters are removed from the active optimization or sampling vector and supplied at fixed values during model evaluation. This means the model code can usually be written in the same way regardless of whether a parameter is fixed.

9.3 Limitations Of `fixed`

Use fixed for fixing parameters to numeric values. Equality constraints such as “make mean0 and mean1 equal” are usually better expressed by changing the model parameterization so that both quantities are built from the same underlying parameter.

For example:

parameters = {
  mean_common <- Dim()
}
transform = {
  mean0 <- mean_common
  mean1 <- mean_common
}

9.4 Using Optimization Results As Fixed Values

fit_map <- mdl$optimize(num_estimate = 10)
est <- fit_map$estimate(pars = "parameters", drop = FALSE)

mdl_fixed <- mdl$fixed_model(
  fixed = list(b = est$b)
)

9.5 Using MCMC Results As Fixed Values

Use posterior summaries, not raw draws, as fixed values.

eap <- fit_mcmc$EAP(pars = "parameters", drop = FALSE)
mdl_fixed <- mdl$fixed_model(fixed = list(b = eap$b))

9.6 `fixed` And Bayes Factors

Bayes factors compare models with different parameter spaces. A fixed-parameter model can be useful as a null model, but priors and constraints must be chosen deliberately.

10. `rtmb_code()` Blocks

rtmb_code() separates model code into blocks.

code <- rtmb_code(
  setup = {
    N <- length(Y)
  },
  parameters = {
    mu <- Dim()
    sigma <- Dim(lower = 0)
  },
  transform = {
    z <- (Y - mu) / sigma
  },
  model = {
    Y ~ normal(mu, sigma)
  },
  generate = {
    log_lik <- normal_lpdf(Y, mu, sigma, sum = FALSE)
    report(log_lik)
  }
)

Block	Role
`setup`	Data preprocessing and helper definitions outside AD
`parameters`	Estimated parameter declarations
`transform`	Deterministic quantities used by the model and ADREPORT/SE
`model`	Likelihood and priors
`generate`	Post-estimation quantities such as predictions and `log_lik`

10.1 What Belongs In `setup`

Use setup for calculations that do not need AD:

Dimensions and indexes.
Missing-data preprocessing.
Design matrices.
Group indexes.
Helper functions that operate on data only.

setup = {
  N <- nrow(df)
  X <- model.matrix(~ x1 + x2, df)
  K <- ncol(X)
}

10.2 What Belongs In `transform`

Use transform for derived quantities that depend on parameters and may need standard errors or ADREPORT-style treatment.

transform = {
  eta <- Intercept + X %*% b
  p <- inv_logit(eta)
  report(p)
}

10.3 What Belongs In `generate`

Use generate for quantities computed after estimation, such as predictions, correlations, posterior predictive summaries, and pointwise log-likelihoods.

generate = {
  y_rep_mean <- mu
  log_lik <- normal_lpdf(Y, mu, sigma, sum = FALSE)
  report(y_rep_mean, log_lik)
}

10.4 Choosing Between `transform` And `generate`

Use transform when the quantity should participate in AD-based reporting, standard errors, or model evaluation. Use generate when the quantity is a post-estimation output. The printed code may look similar, but internally the two blocks serve different purposes.

11. Parameter Declarations And Types

Parameters are declared with Dim().

parameters = {
  alpha <- Dim()
  b <- Dim(K)
  L <- Dim(c(P, P), type = "CF_corr")
  sigma <- Dim(lower = 0)
}

11.1 Dimensions

Declaration	Shape
`Dim()`	Scalar
`Dim(K)`	Vector of length `K`
`Dim(c(R, C))`	Matrix-like object `R x C`
`Dim(c(A, B, C))`	Array-like object

11.2 Constraints

Argument	Meaning
`lower = 0`	Positive parameter
`upper = 1`	Upper-bounded parameter
`lower = 0, upper = 1`	Unit-interval parameter
`type = "simplex"`	Simplex parameter
`type = "ordered"`	Ordered vector
`type = "CF_corr"`	Cholesky factor for correlation matrices

11.3 `random = TRUE`

Use random = TRUE for latent variables or random effects to be marginalized by Laplace approximation in optimization.

parameters = {
  u <- Dim(J, random = TRUE)
  tau <- Dim(lower = 0)
}

12. Distribution Overview

Distributions can be used with Stan-like sampling statements or with explicit log-density functions.

Y ~ normal(mu, sigma)
lp <- lp + normal_lpdf(Y, mu, sigma)

12.1 Continuous Distributions

Common continuous distributions include:

Function family	Example
Normal	`normal_lpdf(y, mu, sigma)`
Student t	`student_t_lpdf(y, df, mu, sigma)`
Exponential	`exponential_lpdf(x, rate)`
Gamma	`gamma_lpdf(x, shape, rate)`
Beta	`beta_lpdf(x, a, b)`
Laplace	`laplace_lpdf(x, mu, sigma)`

12.2 Discrete Distributions

Common discrete distributions include Bernoulli, binomial, Poisson, categorical, ordered-logistic, and negative-binomial families.

y ~ bernoulli_logit(eta)
y ~ poisson(lambda)
y ~ ordered_logistic(eta, cutpoints)

12.3 Multivariate And Matrix Distributions

Use multivariate distributions for correlated outcomes and random effects.

Y ~ multi_normal_CF(mean = mu, sd = sigma, CF_Omega = L_corr)
L_corr ~ lkj_CF_corr(1)

12.4 Mixtures And Special Distributions

Mixture models usually build a matrix of component log densities and combine them with log_sum_exp().

log_dens_mat <- matrix(mu[1] * 0, N, K)
for (k in 1:K) {
  log_dens_mat[, k] <- normal_lpdf(Y, mu[k], sigma[k], sum = FALSE)
}
lp <- lp + sum(log_sum_exp(t(t(log_dens_mat) + log(theta))))

13. Math And Stabilization Functions

BayesRTMB provides helper functions for stable model code.

Function	Purpose
`log_sum_exp()`	Stable log-sum-exp, including AD values
`softmax()`	Softmax probabilities
`log_softmax()`	Log-softmax
`inv_logit()`	Logistic inverse
`logit()`	Logit transform
`log1p_exp()`	Stable `log(1 + exp(x))`
`rtmb_vector()`	Mutable AD-compatible vector
`rtmb_array()`	Mutable AD-compatible array
`distance()`	Euclidean distance
`squared_distance()`	Squared Euclidean distance

When writing categorical models with a baseline category, the natural pattern can be used inside rtmb_code().

log_pi <- log_softmax(c(0, eta))
pi <- softmax(c(0, eta))

14. AD Taping And Performance

RTMB records model evaluation on an automatic-differentiation tape. Code that is simple, vectorized, and AD-compatible tapes faster and evaluates more reliably.

14.1 Use `setup`

Move data-only calculations to setup.

setup = {
  X <- model.matrix(~ x1 + x2, df)
}

14.2 Prefer Vectorization

Prefer vectorized operations when they are clear and AD-compatible.

mu <- Intercept + X %*% b
Y ~ normal(mu, sigma)

14.3 Avoid `apply`-Style Functions In AD Code

Many apply-style functions are convenient but can be difficult to tape inside AD code. Use explicit loops when necessary, especially for AD values.

14.4 Notes On `if` And `ifelse`

Avoid branching on parameter-dependent quantities. Branching on data or setup values is usually fine.

# Good: branch decided by data/setup
if (family == "gaussian") {
  Y ~ normal(mu, sigma)
}

14.5 `rtmb_vector()` And `rtmb_array()`

Use rtmb_vector() and rtmb_array() when you need a mutable container that will receive AD values inside a loop.

log_lik <- rtmb_vector(0, N)
for (i in 1:N) {
  log_lik[i] <- normal_lpdf(Y[i], mu[i], sigma)
}

For matrices or arrays:

A <- rtmb_array(0, dim = c(N, K))
for (k in 1:K) {
  A[, k] <- normal_lpdf(Y, mu[k], sigma[k], sum = FALSE)
}

14.6 Taping Time And AD Evaluation Speed

Long loops, large generated quantities, and repeated matrix decompositions can increase taping time. Keep generated quantities focused on values that you will use.

14.7 Patterns To Avoid Inside AD Tapes

Avoid these patterns inside parameter-dependent code when possible:

numeric(N) followed by assignment of AD values.
matrix(0, ...) followed by elementwise AD assignment.
apply() over AD arrays.
Branching on parameter-dependent conditions.
Recomputing data-only indexes in the model block.

15. Common Recipes

15.1 Get Only Estimates From MCMC

fit <- mdl$sample()
fit$EAP(pars = "parameters")
fit$MAP(pars = "parameters")

15.2 Use MAP Initial Values For MCMC

fit_map <- mdl$optimize(num_estimate = 20)
init <- fit_map$estimate(pars = "parameters", drop = FALSE)
fit_mcmc <- mdl$sample(init = init)

15.3 Extract Only Generated Quantities

fit$estimate(pars = "generate")
fit_mcmc$generated_quantities()

15.4 Extract Draws Including Random Effects

fit_mcmc$draws(include_random = TRUE)

15.5 Check VB Convergence

fit_vb <- mdl$variational(num_estimate = 5)
fit_vb$plot_elbo()
fit_vb$diagnose()

15.6 Compare Models With Classic Estimation

fit1 <- rtmb_lm(y ~ x1, data = dat)$classic()
fit2 <- rtmb_lm(y ~ x1 + x2, data = dat)$classic()

anova(fit1, fit2)
AIC(fit1, fit2)
BIC(fit1, fit2)

15.7 Compare Coefficient Inclusion With A Bayes Factor

mdl_full <- rtmb_lm(y ~ x1 + x2, data = dat, prior = prior_normal())
mdl_null <- rtmb_lm(y ~ x1, data = dat, prior = prior_normal())

fit_full <- mdl_full$sample()
fit_null <- mdl_null$sample()

fit_full$bayes_factor(fit_null)

15.8 Build A Fixed Model Explicitly

mdl_fixed <- mdl$fixed_model(
  fixed = list(b = c(0, 0))
)

15.9 Upgrade An Old Fit Object

Use upgrade_fit() when you want to reuse a saved fit object with the current class definitions after updating the package.

fit <- readRDS("old-fit.rds")
fit <- upgrade_fit(fit)

If the embedded model object also needs current methods, upgrade it as well.

fit <- upgrade_fit(fit, model = TRUE)

15.10 Run MCMC In Parallel

fit <- mdl$sample(
  chains = 4,
  parallel = TRUE,
  sampling = 1000,
  warmup = 1000
)

Parallel workers need access to the functions and data used by the model. Wrapper models preserve the setup environment needed by generated code.

16. Caveats

Check MCMC diagnostics before interpreting posterior summaries.
Use MCMC rather than VB for final uncertainty summaries when the model is difficult or highly nonlinear.
Use WAIC = TRUE before fitting if you plan to call WAIC().
Ensure fixed values match the internal parameter names and dimensions.
For equality constraints, prefer reparameterizing the model instead of trying to express the constraint through fixed.
Keep data-only calculations in setup and parameter-dependent calculations in transform, model, or generate as appropriate.
Use rtmb_vector() and rtmb_array() for mutable AD containers.
When a wrapper is unclear, inspect the generated code with mdl$print_code().

BayesRTMB Analysis Reference