# sdmTMB model description

# Introduction

This vignette describes the statistical model underlying sdmTMB. Further details are available in the sdmTMB paper (Anderson et al. 2024), which should be referenced and cited when using sdmTMB in a publication.

# Other vignettes available

If this vignette is being viewed on CRAN, note that many other vignettes describing how to use sdmTMB are available on the documentation site under Articles.

# Notation conventions

This appendix uses the following notation conventions, which generally follow the guidance in Edwards & Auger-Méthé (2019):

- Greek symbols for parameters,

- the Latin/Roman alphabet for data except for cases where another symbol is used by convention,

- bold symbols for vectors or matrices (e.g., \(\boldsymbol{\omega}\) is a vector and \(\omega_{\mathbf{s}}\) is the value of \(\boldsymbol{\omega}\) at point in space \(\mathbf{s}\)), with Latin/Roman symbols in non-italics (e.g., \(\mathbf{X}\), \(\mathbf{Q}\)),

- \(\phi\) for all distribution dispersion parameters for consistency with the code,

- \(\mathbb{E}[y]\) to define the expected value (mean) of variable \(y\),

- \(\mathrm{Var}[y]\) to define the expected variance of the variable \(y\),

- a \(^*\) superscript represents interpolated or projected values as opposed to values at knot locations (e.g., \(\boldsymbol{\omega}\) vs. \(\boldsymbol{\omega}^*\)), and

- where possible, notation has been chosen to match VAST (Thorson 2019) to maintain consistency (e.g., \(\boldsymbol{\omega}\) for spatial fields and \(\boldsymbol{\epsilon}_t\) for spatiotemporal fields).

Tables of indices and symbols are summarized in notation reference tables at the end of this vignette.

# sdmTMB model structure

The complete sdmTMB model can be written as

\[ \begin{aligned} \eta_{\mathbf{s},t} &= \mathbf{X}^{\mathrm{main}}_{\mathbf{s},t} \boldsymbol{\beta}+ O_{\mathbf{s},t} + \mathbf{Z}_{g} \mathbf{b}_{g} + \mathbf{X}^{\mathrm{tvc}}_{\mathbf{s},t} \boldsymbol{\gamma}_{t} + \mathbf{X}^{\mathrm{svc}}_{\mathbf{s},t} \boldsymbol{\zeta}_{\mathbf{s}} + \omega_{\mathbf{s}} + \epsilon_{\mathbf{s},t},\\ \mu_{\mathbf{s},t} &= f^{-1} \left( \eta_{\mathbf{s},t} \right),\\ \mathbb{E}[y_{\mathbf{s},t}] &= \mu_{\mathbf{s},t}, \end{aligned} \]

where

- \(y_{\mathbf{s},t}\) represents the response data at point \(\mathbf{s}\) and time \(t\);
- \(\eta\) represents the linear predictor (i.e., in link space)
- \(\mu\) represents the conditional mean on the response/data scale (after applying the inverse link);
- \(f\) represents a link function (e.g., log or logit) and \(f^{-1}\) represents its inverse, and \(\eta\) represents the linear predictor before applying \(f^{-1}\);
- \(\mathbf{X}^{\mathrm{main}}\), \(\mathbf{X}^{\mathrm{tvc}}\), and \(\mathbf{X}^{\mathrm{svc}}\) represent design matrices (the superscript identifiers ‘main’ = main effects, ‘tvc’ = time varying coefficients, and ‘svc’ = spatially varying coefficients);
- \(\boldsymbol{\beta}\) represents a vector of fixed-effect coefficients;
- \(O_{\mathbf{s},t}\) represents an offset: a covariate (usually log transformed) with a coefficient fixed at one that enters on the linear predictor scale (before the inverse link);
- \(\mathbf{Z}_{g}\) represents the random-effect design row(s) for group \(g\) (a subset of the matrix \(\mathbf{Z}\)) corresponding to \(\mathbf{b}_{g}\); these random effects enter on the linear predictor scale; the scalar special case is a random intercept \(\alpha_{g}\sim \mathrm{N}(0,\sigma_\alpha^2)\), and more generally random slopes and intercepts are vector-valued with full covariance as described below;
- \(\boldsymbol{\gamma}_{t}\) represents a vector of time-varying coefficients, where each coefficient \(\gamma_{p,t}\) can follow a random walk or AR(1) process;
- \(\boldsymbol{\zeta}_{\mathbf{s}}\) represents a vector of spatially varying coefficients (random fields), where each coefficient \(\zeta_{l,\mathbf{s}} \sim \mathrm{MVN}(\boldsymbol{0},\boldsymbol{\Sigma}_{\zeta,l})\);
- \(\omega_{\mathbf{s}}\) represents a spatial component (a Gaussian Markov random field, GMRF), \(\omega_{\mathbf{s}} \sim \mathrm{MVN}(\boldsymbol{0},\boldsymbol{\Sigma}_\omega)\) with Matérn covariance built from the mesh; and
- \(\epsilon_{\mathbf{s},t}\) represents a spatiotemporal component (a GMRF), \(\epsilon_{\mathbf{s},t} \sim \mathrm{MVN}(\boldsymbol{0},\boldsymbol{\Sigma}_{\epsilon})\) with Matérn covariance over space and temporal evolution as specified.

A single sdmTMB model will rarely, if ever, contain all of the above components. Next, we will split the model to describe the various parts in more detail using ‘\(\ldots\)’ to represent the other optional components.

## Main effects

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \mathbf{X}^{\mathrm{main}}_{\mathbf{s},t} \boldsymbol{\beta}+ \ldots \right) \end{aligned} \]

Within `sdmTMB()`, \(\mathbf{X}^{\mathrm{main}}_{\mathbf{s},t} \boldsymbol{\beta}\) is defined by the `formula` argument and represents the main-effect model matrix and a corresponding vector of coefficients. This main effect formula can contain optional penalized smoothers or non-linear functions as defined below.

### Smoothers

Smoothers in sdmTMB are implemented with the same formula syntax familiar to mgcv (Wood 2017) users fitting GAMs (generalized additive models). Smooths are implemented in the formula using `+ s(x)`, which implements a smooth from `mgcv::s()`. Within these smooths, the same syntax commonly used in `mgcv::s()` can be applied, e.g. 2-dimensional smooths may be constructed with `+ s(x, y)`; smooths can be specific to various factor levels, `+ s(x, by = group)`; smooths can vary according to a continuous variable, `+ s(x, by = x2)`; the basis function dimensions may be specified, e.g. `+ s(x, k = 4)` (see `?mgcv::choose.k`); and various types of splines may be constructed such as cyclic splines to model seasonality, e.g. `+ s(month, bs = "cc", k = 12)`.

While mgcv can fit unpenalized (e.g., B-splines) or penalized splines (P-splines), sdmTMB only implements penalized splines. The penalized splines are constructed in sdmTMB using the function `mgcv::smooth2random()`, which transforms splines into random effects (and associated design matrices) that are estimable in a mixed-effects modelling framework. This is the same approach as is implemented in the R packages gamm4 (Wood & Scheipl 2020) and brms (Bürkner 2017).

### Linear break-point threshold models

The linear break-point or “hockey stick” model can be used to describe threshold or asymptotic responses. This function consists of two pieces, so that for \(x < b_{1}\), \(s(x) = x \cdot b_{0}\), and for \(x > b_{1}\), \(s(x) = b_{1} \cdot b_{0}\). In both cases, \(b_{0}\) represents the slope of the function up to a threshold, and the product \(b_{1} \cdot b_{0}\) represents the value at the asymptote. No constraints are placed on parameters \(b_{0}\) or \(b_{1}\).

These models can be fit by including `+ breakpt(x)` in the model formula, where `x` is a covariate. The formula can contain a single break-point covariate.

### Logistic threshold models

Models with logistic threshold relationships between a predictor and the response can be fit with the form

\[ s(x)=\tau + \psi\ { \left[ 1+{ e }^{ -\ln \left(19\right) \cdot \left( x-s50 \right) / \left(s95 - s50 \right) } \right] }^{-1}, \]

where \(s\) represents the logistic function, \(\psi\) is a scaling parameter (controlling the height of the y-axis for the response; unconstrained), \(\tau\) is an intercept, \(s50\) is a parameter controlling the point at which the function reaches 50% of the maximum (\(\psi\)), and \(s95\) is a parameter controlling the point at which the function reaches 95% of the maximum. The parameter \(s50\) is unconstrained but \(s95\) is constrained to be larger than \(s50\).

These models can be fit by including `+ logistic(x)` in the model formula, where `x` is a covariate. The formula can contain a single logistic covariate.

## Spatial random fields

Spatial random fields, \(\omega_{\mathbf{s}}\), are included if `spatial = 'on'` (or `TRUE`) and omitted if `spatial = 'off'` (or `FALSE`).

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \omega_{\mathbf{s}} + \ldots \right),\\ \boldsymbol{\omega}&\sim \mathrm{MVNormal} \left( \boldsymbol{0}, \boldsymbol{\Sigma}_\omega \right),\\ \end{aligned} \] The marginal standard deviation of \(\boldsymbol{\omega}\) is indicated by `Spatial SD` in the printed model output or as `sigma_O` in the output of `sdmTMB::tidy(fit, "ran_pars")`. The ‘O’ is for ‘omega’ (\(\omega\)).

Internally, the random fields follow a Gaussian Markov random field (GMRF)

\[ \boldsymbol{\omega}\sim \mathrm{MVNormal}\left(\boldsymbol{0}, \sigma_\omega^2 \mathbf{Q}^{-1}_\omega\right), \] where \(\mathbf{Q}_\omega\) is a sparse precision matrix and \(\sigma_\omega^2\) is the marginal variance.

## Spatiotemporal random fields

Spatiotemporal random fields are included by default if there are multiple time elements (`time` argument is not `NULL`) and can be set to IID (independent and identically distributed, `'iid'`; default), AR(1) (`'ar1'`), random walk (`'rw'`), or off (`'off'`) via the `spatiotemporal` argument. These text values are case insensitive.

Spatiotemporal random fields are represented by \(\boldsymbol{\epsilon}_t\) within sdmTMB. This has been chosen to match the representation in VAST (Thorson 2019). The marginal standard deviation of \(\boldsymbol{\epsilon}_t\) is indicated by `Spatiotemporal SD` in the printed model output or as `sigma_E` in the output of `sdmTMB::tidy(fit, "ran_pars")`. The ‘E’ is for ‘epsilon’ (\(\epsilon\)).

### IID spatiotemporal random fields

IID spatiotemporal random fields (`spatiotemporal = 'iid'`) can be represented as

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \epsilon_{\mathbf{s},t} + \ldots \right),\\ \boldsymbol{\epsilon}_{t} &\sim \mathrm{MVNormal} \left( \boldsymbol{0}, \boldsymbol{\Sigma}_{\epsilon} \right). \end{aligned} \]

where \(\epsilon_{\mathbf{s},t}\) represent random field deviations at point \(\mathbf{s}\) and time \(t\). The random fields are assumed independent across time steps.

Similarly to the spatial random fields, these spatiotemporal random fields (including all versions described below) are parameterized internally with a sparse precision matrix (\(\mathbf{Q}_\epsilon\))

\[ \boldsymbol{\epsilon}_{t} \sim \mathrm{MVNormal}\left(\boldsymbol{0}, \sigma_\epsilon^2 \mathbf{Q}^{-1}_\epsilon\right). \]

### AR(1) spatiotemporal random fields

First-order auto regressive, AR(1), spatiotemporal random fields (`spatiotemporal = 'ar1'`) add a parameter defining the correlation between random field deviations from one time step to the next. They are defined as

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \delta_{\mathbf{s},t} + \ldots \right),\\ \boldsymbol{\delta}_{t=1} &\sim \mathrm{MVNormal} (\boldsymbol{0}, \boldsymbol{\Sigma}_{\epsilon}),\\ \boldsymbol{\delta}_{t>1} &= \rho \boldsymbol{\delta}_{t-1} + \sqrt{1 - \rho^2} \boldsymbol{\epsilon}_{t}, \: \boldsymbol{\epsilon}_{t} \sim \mathrm{MVNormal} \left(\boldsymbol{0}, \boldsymbol{\Sigma}_{\epsilon} \right), \end{aligned} \] where \(\rho\) is the correlation between subsequent spatiotemporal random fields. The \(\rho \boldsymbol{\delta}_{t-1} + \sqrt{1 - \rho^2}\) term scales the spatiotemporal variance by the correlation such that it represents the steady-state marginal variance. The correlation \(\rho\) allows for mean-reverting spatiotemporal fields, and is constrained to be \(-1 < \rho < 1\). Internally, the parameter is estimated as `ar1_phi`, which is unconstrained. The parameter `ar1_phi` is transformed to \(\rho\) with \(\rho = 2 \left( \mathrm{logit}^{-1}(\texttt{ar1\_phi}) \right) - 1\), mapping `ar1_phi` to the range \((-1, 1)\).

### Random walk spatiotemporal random fields (RW)

Random walk spatiotemporal random fields (`spatiotemporal = 'rw'`) represent a model where the difference in spatiotemporal deviations from one time step to the next are IID. They are defined as

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \delta_{\mathbf{s},t} + \ldots \right),\\ \boldsymbol{\delta}_{t=1} &\sim \mathrm{MVNormal} (\boldsymbol{0}, \boldsymbol{\Sigma}_{\epsilon}),\\ \boldsymbol{\delta}_{t>1} &= \boldsymbol{\delta}_{t-1} + \boldsymbol{\epsilon}_{t}, \: \boldsymbol{\epsilon}_{t} \sim \mathrm{MVNormal} \left(\boldsymbol{0}, \boldsymbol{\Sigma}_{\epsilon} \right), \end{aligned} \]

where the distribution of the spatiotemporal field in the initial time step is the same as for the AR(1) model, but the absence of the \(\rho\) parameter allows the spatiotemporal field to be non-stationary in time. Note that, in contrast to the AR(1) parametrization, the variance is no longer the steady-state marginal variance.

## Time-varying regression parameters

Parameters can be modelled as time-varying according to a random walk or first-order autoregressive, AR(1), process. The time-series model is defined by `time_varying_type`. For all types:

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \mathbf{X}^{\mathrm{tvc}}_{\mathbf{s},t} \boldsymbol{\gamma}_{t} + \ldots \right), \end{aligned} \] where \(\boldsymbol{\gamma}_{t}\) is an optional vector of time-varying regression parameters and \(\mathbf{X}^{\mathrm{tvc}}_{\mathbf{s},t}\) is the corresponding model matrix with covariate values. This is defined via the `time_varying` argument, assuming that the `time` argument is also supplied a column name. `time_varying` takes a one-sided formula. `~ 1` implies a time-varying intercept.

The following equations describe the time-series process for a single time-varying coefficient \(\gamma_{p,t}\), where \(p\) indexes the time-varying coefficient. When multiple time-varying coefficients are specified, each follows its time-series process independently with its own variance \(\sigma_{\gamma,p}^2\) and (for AR1) correlation parameter \(\rho_{\gamma,p}\).

For `time_varying_type = 'rw'`, the first time step is estimated independently:

\[ \begin{aligned} \gamma_{p,t=1} &\sim \mathrm{Uniform} \left(-\infty, \infty \right) \text{ (treated as an unconstrained parameter)},\\ \gamma_{p,t>1} &\sim \mathrm{Normal} \left(\gamma_{p,t-1}, \sigma_{\gamma,p}^2 \right). \end{aligned} \]

In this case, the first time-step value is given an implicit uniform prior. I.e., the same variable should not appear in the fixed effect formula since the initial value is estimated as part of the time-varying formula. The formula `time_varying = ~ 1` implicitly represents a time-varying intercept (assuming the `time` argument has been supplied) and, this case, the intercept should be omitted from the main effects (`formula ~ + 0 + ...` or `formula ~ -1 + ...`).

For `time_varying_type = 'rw0'`, the first time step is estimated from a mean-zero prior:

\[ \begin{aligned} \gamma_{p,t=1} &\sim \mathrm{Normal} \left(0, \sigma_{\gamma,p}^2 \right),\\ \gamma_{p,t>1} &\sim \mathrm{Normal} \left(\gamma_{p,t-1}, \sigma_{\gamma,p}^2 \right). \end{aligned} \] In this case, the time-varying variable (including the intercept) should be included in the main effects.

For `time_varying_type = 'ar1'`:

\[ \begin{aligned} \gamma_{p,t=1} &\sim \mathrm{Normal} \left(0, \sigma_{\gamma,p}^2 \right),\\ \gamma_{p,t>1} &\sim \mathrm{Normal} \left(\rho_{\gamma,p}\gamma_{p,t-1}, (1 - \rho_{\gamma,p}^2) \sigma_{\gamma,p}^2 \right), \end{aligned} \] where \(\rho_{\gamma,p}\) is the correlation between subsequent time steps. The first time step is given a mean-zero prior.

## Spatially varying coefficients (SVC)

Spatially varying coefficient models are defined as

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \mathbf{X}^{\mathrm{svc}}_{\mathbf{s}, t} \boldsymbol{\zeta}_{\mathbf{s}} + \ldots \right),\\ \zeta_{l,\mathbf{s}} &\sim \mathrm{MVNormal} \left( \boldsymbol{0}, \boldsymbol{\Sigma}_{\zeta,l} \right), \end{aligned} \]

where \(\zeta_{l,\mathbf{s}}\) represents the \(l\)-th spatially varying coefficient field at location \(\mathbf{s}\). When multiple spatially varying coefficients are specified, each is estimated as an independent spatial random field with its own marginal variance \(\sigma_{\zeta,l}^2\). Usually, \(\mathbf{X}^{\mathrm{svc}}_{\mathbf{s}, t}\) would represent a prediction matrix that is constant spatially for a given time \(t\) as defined by a one-sided formula supplied to `spatial_varying`. For example `spatial_varying = ~ 0 + x`, where `0` omits the intercept.

The random fields are parameterized internally with a sparse precision matrix (\(\mathbf{Q}_\zeta\))

\[ \zeta_{l,\mathbf{s}} \sim \mathrm{MVNormal}\left(\boldsymbol{0}, \sigma_{\zeta,l}^2 \mathbf{Q}^{-1}_\zeta\right). \]

## Random intercepts and slopes

Multilevel (hierarchical) intercepts and slopes follow the familiar lme4/glmmTMB formulation. A single grouping factor can carry an intercept-only term or a vector of random effects (intercept plus slopes) with a full covariance:

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + \mathbf{Z}_{g} \mathbf{b}_{g} + \ldots \right),\\ \mathbf{b}_{g} &\sim \mathrm{MVNormal} \left( \boldsymbol{0}, \boldsymbol{\Sigma}_{g} \right), \end{aligned} \]

where \(\mathbf{Z}_{g}\) is the design row(s) for group \(g\) (e.g., intercept and covariate values) and \(\mathbf{b}_{g}\) is the corresponding vector of random effects for that group. The scalar special case \((1|g)\) corresponds to \(n=1\) with \(\alpha_{g} \sim \mathrm{Normal}(0, \sigma_{\alpha}^2)\).

Use lme4-style syntax in `formula`, e.g. `(1 | g)` for random intercepts, `(1 + x | g)` for correlated intercepts and slopes, or `(1 | g1) + (1 + x | g2)` to give each grouping factor its own (co)variance.

Internally, standard deviations are estimated on the log scale, and correlations are represented by unconstrained Cholesky parameters (via `UNSTRUCTURED_CORR`), yielding a positive-definite \(\boldsymbol{\Sigma}_{g}\) per grouping factor.

## Offset terms

Offset terms can be included through the `offset` argument in `sdmTMB()`. These are included in the linear predictor as

\[ \begin{aligned} \mu_{\mathbf{s},t} &= f^{-1} \left( \ldots + O_{\mathbf{s},t} + \ldots \right), \end{aligned} \]

where \(O_{\mathbf{s},t}\) is an offset term—a log transformed variable without a coefficient (assuming a log link). The offset is not included in the prediction. Therefore, if `offset` represents a measure of effort, for example, the prediction is for one unit of effort (`log(1) = 0`).

# Observation model families

Here we describe the main observation families that are available in sdmTMB and comment on their parametrization, statistical properties, utility, and code representation in sdmTMB. Families are grouped by outcome type to make it easier to locate an appropriate observation model.

## Bounded or binary outcomes (0–1, proportions)

### Binomial

\[ \operatorname{Binomial} \left(N, \mu \right) \] where \(N\) is the size or number of trials, and \(\mu\) is the probability of success for each trial. If \(N = 1\), the distribution becomes the Bernoulli distribution. Internally, the distribution is parameterized as the robust version in TMB, which is numerically stable when probabilities approach 0 or 1. Following the structure of `stats::glm()`, lme4, and glmmTMB, a binomial family can be specified in one of 4 ways:

1. the response may be a factor (and the model classifies the first level versus all others)
2. the response may be binomial (0/1)
3. the response can be a matrix of form `cbind(success, failure)`, or
4. the response may be the observed proportions, and the `weights` argument is used to specify the Binomial size (\(N\)) parameter (`probability ~ ..., weights = N`).

Code defined within TMB.

Example: `family = binomial(link = "logit")`

### Beta-binomial

\[ \operatorname{BetaBinomial} \left(N, \mu, \phi \right) \] where \(N\) is the number of trials, \(\mu\) is the mean success probability, and \(\phi\) is a precision parameter that controls overdispersion relative to a Binomial. The implied Beta parameters are \(\alpha = \mu \phi\) and \(\beta = (1 - \mu)\phi\), and the variance is $ [y] = N (1 - ), , $ which exceeds the Binomial variance unless \(\phi \to \infty\). Available links are logit and cloglog. This family is useful for overdispersed counts of successes/failures (e.g., aggregated Bernoulli data, proportions with extra-binomial variation).

Code defined within sdmTMB and implemented in the TMB likelihood at `src/sdmTMB.cpp` (e.g., lines 974–982).

Example: `family = betabinomial(link = "logit")`

### Beta

\[ \operatorname{Beta} \left(\mu \phi, (1 - \mu) \phi \right) \] where \(\mu\) is the mean and \(\phi\) is a precision parameter. This parametrization follows Ferrari & Cribari-Neto (2004) and the betareg R package (Cribari-Neto & Zeileis 2010). The variance is \(\mu (1 - \mu) / (\phi + 1)\).

Code defined within TMB.

Example: `family = Beta(link = "logit")`

## Count data

### Poisson

\[ \operatorname{Poisson} \left( \mu \right) \] where \(\mu\) represents the mean and \(\mathrm{Var}[y] = \mu\).

Code defined within TMB.

Example: `family = poisson(link = "log")`

### Negative Binomial 2 (NB2)

\[ \operatorname{NB2} \left( \mu, \phi \right) \]

where \(\mu\) is the mean and \(\phi\) is the dispersion parameter. The variance scales quadratically with the mean \(\mathrm{Var}[y] = \mu + \mu^2 / \phi\) (Hilbe 2011). The NB2 parametrization is more commonly seen in ecology than the NB1. Internally, the distribution is parameterized as the robust version in TMB.

Code defined within TMB.

Example: `family = nbinom2(link = "log")`

### Negative Binomial 1 (NB1)

\[ \operatorname{NB1} \left( \mu, \phi \right) \]

where \(\mu\) is the mean and \(\phi\) is the dispersion parameter. The variance scales linearly with the mean \(\mathrm{Var}[y] = \mu + \mu / \phi\) (Hilbe 2011). Internally, the distribution is parameterized as the robust version in TMB.

Code defined within sdmTMB based on NB2 and borrowed from glmmTMB.

Example: `family = nbinom1(link = "log")`

### Negative binomial 2 mixture

This is a 2 component mixture that extends the NB2 distribution, following mixture-distribution approaches for shoaling/aggregation in survey data (Thorson et al. 2011).

\[ (1 - p) \cdot \operatorname{NB2} \left( \mu_1, \phi \right) + p \cdot \operatorname{NB2} \left( \mu_2, \phi \right) \]

where \(\mu_1\) is the mean of the first (smaller component) of the distribution, \(\mu_2\) is the mean of the larger component, and \(p\) controls the contribution of each component to the mixture.

Example: `family = nbinom2_mix(link = "log")`

## Positive continuous outcomes

### Gamma

\[ \operatorname{Gamma} \left( \phi, \frac{\mu}{\phi} \right) \] where \(\phi\) represents the Gamma shape and \(\mu / \phi\) represents the scale. The mean is \(\mu\) and variance is \(\mu^2 / \phi\).

Code defined within TMB.

Example: `family = Gamma(link = "log")`

### Lognormal

sdmTMB uses the “bias-corrected” lognormal distribution where \(\phi\) represents the standard deviation in log-space:

\[ \operatorname{Lognormal} \left( \log \mu - \frac{\phi^2}{2}, \phi^2 \right). \] Because of the bias correction, \(\mathbb{E}[y] = \mu\) and \(\mathrm{Var}[\log y] = \phi^2\).

Code defined within sdmTMB based on the TMB `dnorm()` normal density.

Example: `family = lognormal(link = "log")`

### Generalized gamma

\[ \operatorname{GenGamma} \left( \mu, \phi, Q^{\mathrm{gg}}\right) \]

sdmTMB implements the Prentice (1974) parameterization introduced for spatiotemporal models and index standardization by Dunic et al. (2025), with parameters mean \(\mu\), scale \(\phi\) (standard deviation on the log scale), and a shape parameter \(Q^{\mathrm{gg}}\) (reported as `Generalized gamma Q`).

Here, \(\mu\) is the mean on the data scale, \(\phi\) is the log-scale standard deviation, and \(Q^{\mathrm{gg}}\) controls the shape (\(Q^{\mathrm{gg}}\to 0\) yields the lognormal; \(Q^{\mathrm{gg}}= \phi\) yields the gamma). See Dunic et al. (2025) for the full PDF in the Prentice formulation as implemented. Links available: identity, log, inverse. This flexibility is useful for right-skewed positive responses with tails heavier or lighter than gamma/lognormal, and is often paired in a hurdle/mixture as `delta_gengamma()` for zero-inflated biomass or catch data (see Dunic et al. (2025)).

Code defined within sdmTMB.

Example: `family = gengamma(link = "log")`; delta/hurdle: `family = delta_gengamma(link1 = "logit", link2 = "log")`.

### Tweedie

\[ \operatorname{Tweedie} \left(\mu, p, \phi \right), \: 1 < p < 2 \]

where \(\mu\) is the mean, \(p\) is the power parameter constrained between 1 and 2, and \(\phi\) is the dispersion parameter. The Tweedie distribution can be helpful for modelling data that are positive and continuous but also contain zeros.

Internally, \(p\) is transformed from \(\mathrm{logit}^{-1} (\texttt{thetaf}) + 1\) to constrain it between 1 and 2 and is estimated as an unconstrained variable.

The source code is implemented as in the cplm package (Zhang 2013) and is based on Dunn & Smyth (2005). The TMB version is defined here.

Example: `family = tweedie(link = "log")`

### Gamma mixture

This is a 2 component mixture that extends the Gamma distribution, motivated by mixture-distribution treatments of aggregation in survey data (Thorson et al. 2011),

\[ (1 - p) \cdot \operatorname{Gamma} \left( \phi, \frac{\mu_{1}}{\phi} \right) + p \cdot \operatorname{Gamma} \left( \phi, \frac{\mu_{2}}{\phi} \right), \] where \(\phi\) represents the Gamma shape, \(\mu_{1} / \phi\) represents the scale for the first (smaller component) of the distribution, \(\mu_{2} / \phi\) represents the scale for the second (larger component) of the distribution, and \(p\) controls the contribution of each component to the mixture (also interpreted as the probability of larger events).

The mean is \((1-p) \cdot \mu_{1} + p \cdot \mu_{2}\). The variance follows the usual mixture formula: \((1-p)\left(\mu_1^2 / \phi + \mu_1^2\right) + p\left(\mu_2^2 / \phi + \mu_2^2\right) - \left[(1-p)\mu_1 + p\mu_2\right]^2\).

Here, and for the other mixture distributions, the probability of the larger mean can be obtained from `plogis(fit$model$par[["logit_p_extreme"]])` and the ratio of the larger mean to the smaller mean can be obtained from `1 + exp(fit$model$par[["log_ratio_mix"]])`. The standard errors are available in the TMB sdreport: `fit$sd_report`.

If you wish to fix the probability of a large (i.e., extreme) mean, which can be hard to estimate, you can fix this value and pass this to the family:

    sdmTMB(...,
      family = gamma_mix(link = "log", p_extreme = 0.01)
    )

See also `family = delta_gamma_mix()` for an extension incorporating this distribution with delta models.

### Lognormal mixture

This is a 2 component mixture that extends the lognormal distribution, again in the spirit of mixture approaches for aggregating/ shoaling data (Thorson et al. 2011),

\[ (1 - p) \cdot \operatorname{Lognormal} \left( \log \mu_{1} - \frac{\phi^2}{2}, \phi^2 \right) + p \cdot \operatorname{Lognormal} \left( \log \mu_{2} - \frac{\phi^2}{2}, \phi^2 \right). \]

Because of the bias correction, \(\mathbb{E}[y] = (1-p) \cdot \mu_{1} + p \cdot \mu_{2}\). The log-scale variance of the mixture is not simply \(\phi^2\); it can be obtained with the standard mixture-variance formula using component log-means \(\log \mu_i - \phi^2/2\) and log-variance \(\phi^2\).

As with the Gamma mixture, \(p\) controls the contribution of each component to the mixture (also interpreted as the probability of larger events).

Example: `family = lognormal_mix(link = "log")`. See also `family = delta_lognormal_mix()` for an extension incorporating this distribution with delta models. Like with the gamma mixture, fixed probabilities of extreme events (\(p\) in notation above) can be passed in, e.g.

    sdmTMB(...,
      family = delta_lognormal_mix(p_extreme = 0.01)
    )

## Continuous real-valued outcomes

### Gaussian

\[ \operatorname{Normal} \left( \mu, \phi^2 \right) \] where \(\mu\) is the mean and \(\phi\) is the standard deviation. The variance is \(\phi^2\).

Example: `family = Gaussian(link = "identity")`

Code defined within TMB.

### Student-t

\[ \operatorname{Student-t} \left( \mu, \phi, \nu \right) \]

where \(\nu\), the degrees of freedom (`df`), is a user-supplied fixed parameter. Lower values of \(\nu\) result in heavier tails compared to the Gaussian distribution. Above approximately `df = 20`, the distribution becomes very similar to the Gaussian. The Student-t distribution with a low degrees of freedom (e.g., \(\nu \le 7\)) can be helpful for modelling data that would otherwise be suitable for Gaussian but needs an approach that is robust to outliers (e.g., Anderson et al. 2017).

Code defined within sdmTMB based on the `dt()` distribution in TMB.

Example: `family = student(link = "log", df = 7)`

## Zero-inflated and hurdle structures

### Delta models

sdmTMB allows for several different kinds of delta (also known as hurdle) models. These families are implemented by specifying the family as a delta distribution. For example:

    sdmTMB(
      ...,
      family = delta_gamma()
    )

The list of supported families is included in the documentation on additional families, delta models, and Poisson-link delta models. By default, the `delta_*` families don’t use the Poisson link, but this structure can be specified with `delta_gamma(type = "poisson-link")`, which follows the formulation in Thorson (2018).

In the “standard” delta model implementation, sdmTMB constructs two internal models (formally, two “linear predictors”), with the first model representing presence-absence and the second model representing the positive component (such as catch rates in fisheries applications). Default links associated with each family can be inspected with `delta_lognormal()` and equivalent functions. For the standard delta models, the default first linear predictor link is logit. For the Poisson-link type, the first linear predictor link is log.

The `formula`, `spatial`, `spatiotemporal`, and `share_range` arguments of `sdmTMB()` can be specified independently as a 2-element list. For example, the spatial random field might be estimated for only the first linear predictor with:

    sdmTMB(
      ...,
      family = delta_gamma(),
      spatial = list("on", "off")
    )

Or for we may want separate main-effects formulas. For example:

    sdmTMB(
      formula = list(
        y ~ depth + I(depth^2),
        y ~ 1),
      family = delta_gamma()
    )

All other arguments are shared between the linear predictors.

Currently if delta models contain smoothers, both components must have the same main-effects formula.

# Gaussian random fields

## Matérn parameterization

The Matérn defines the covariance \(\Phi \left( s_j, s_k \right)\) between spatial locations \(s_j\) and \(s_k\) as

\[ \Phi\left( s_j,s_k \right) = \tau^2 \left[ 2^{\nu - 1}\Gamma(\nu) \right]^{-1} (\kappa d_{jk})^\nu K_\nu \left( \kappa d_{jk} \right), \]

where \(\tau^2\) is a scale/precision factor (higher \(\tau\) means lower marginal variance), \(\nu\) controls the smoothness, \(\Gamma\) represents the Gamma function, \(d_{jk}\) represents the distance between locations \(s_j\) and \(s_k\), \(K_\nu\) represents the modified Bessel function of the second kind, and \(\kappa\) represents the decorrelation rate. The parameter \(\nu\) is set to 1 because the SPDE approach that gives us sparse matrices (and therefore major computational speedups) requires \(\alpha = \nu + d/2\) to be an integer when working in 2D (so \(\nu = 1\)) (Lindgren et al. 2011). Internally, the parameters \(\kappa\) and \(\tau\) are converted to range and marginal standard deviation \(\sigma\) as \(\textrm{range} = \sqrt{8} / \kappa\) and \(\sigma = 1 / \sqrt{4 \pi \exp \left(2 \log(\tau) + 2 \log(\kappa) \right) }\), so increasing \(\kappa\) or \(\tau\) decreases \(\sigma\).

In the case of a spatiotemporal model with both spatial and spatiotemporal fields, if `share_range = TRUE` in `sdmTMB()` (the default), then a single \(\kappa\) and range are estimated with separate \(\sigma_\omega\) and \(\sigma_\epsilon\). This often makes sense since data are often only weakly informative about \(\kappa\). If `share_range = FALSE`, then separate \(\kappa_\omega\) and \(\kappa_\epsilon\) are estimated. The spatially varying coefficient field always shares \(\kappa\) with the spatial random field.

## Projection \(\mathbf{A}\) matrix

The values of the spatial variables at the knots are multiplied by a projection matrix \(\mathbf{A}\) that bilinearly interpolates from the knot locations to the values at the locations of the observed or predicted data (Lindgren & Rue 2015)

\[ \boldsymbol{\omega}^* = \mathbf{A}\boldsymbol{\omega}, \] where \(\boldsymbol{\omega}^*\) represents the values of the spatial random fields at the observed locations or predicted data locations. The matrix \(\mathbf{A}\) has a row for each data point or prediction point and a column for each knot. Three non-zero elements on each row define the weight of the neighbouring 3 knot locations for location \(\mathbf{s}\). The same bilinear interpolation happens for any spatiotemporal random fields

\[ \boldsymbol{\epsilon}_t^* = \mathbf{A}\boldsymbol{\epsilon}_t. \]

## Anisotropy

TMB allows for anisotropy, where spatial covariance may be asymmetric with respect to latitude and longitude (full details). Anisotropy can be turned on or off with the logical `anisotropy` argument to `sdmTMB()`. There are a number of ways to implement anisotropic covariance (Fuglstad et al. 2015), and we adopt a 2-parameter rotation matrix \(\mathbf{H}\). The elements of \(\mathbf{H}\) are defined by the parameter vector \(\boldsymbol{x}\) so that \(H_{1,1} = x_{1}\), \(H_{1,2} = H_{2,1} = x_{2}\) and \(H_{2,2} = (1 + x_{2}^2) / x_{1}\).

Once a model is fitted with `sdmTMB()`, the anisotropy relationships may be plotted using the `plot_anisotropy()` function, which takes the fitted object as an argument. If a barrier mesh is used, anisotropy is disabled.

## Incorporating physical barriers into the SPDE

In some cases the spatial domain of interest may be complex and bounded by some barrier such as by land or water (e.g., coastlines, islands, lakes). SPDE models allow for physical barriers to be incorporated into the modelling (Bakka et al. 2019). With `sdmTMB()` models, the mesh construction occurs in two steps: the user (1) constructs a mesh with a call to `sdmTMBextra::make_mesh()`, and (2) passes the mesh to `sdmTMBextra::add_barrier_mesh()`. The barriers must be constructed as `sf` objects (Pebesma 2018) with polygons defining the barriers. See `?sdmTMBextra::add_barrier_mesh` for an example.

The barrier implementation requires the user to select a fraction value (`range_fraction` argument) that defines the fraction of the usual spatial range when crossing the barrier (Bakka et al. 2019). For example, if the range was estimated at 10 km, `range_fraction = 0.2` would assign a 2 km range to the triangles marked as barrier. That makes correlation decay much faster through the barrier than around it, which effectively discourages spatial smoothing from “jumping” across landmasses. From experimentation, values around 0.1 or 0.2 seem to work well but values much lower than 0.1 can result in convergence issues.

This website by Francesco Serafini and Haakon Bakka provides an illustration with INLA. The implementation within TMB was borrowed from code written by Olav Nikolai Breivik and Hans Skaug at the TMB Case Studies Github site.

# Optimization

## Optimization details

The sdmTMB model is fit by maximum marginal likelihood. Internally, a TMB (Kristensen et al. 2016) model template calculates the marginal log likelihood and its gradient, and the negative log likelihood is minimized via the non-linear optimization routine `stats::nlminb()` in R (Gay 1990; R Core Team 2021). Random effects are represented by the values that maximize the log likelihood conditional on the fixed effects (their conditional modes), and the Laplace approximation integrates over them when evaluating the likelihood (Kristensen et al. 2016).

Like AD Model Builder (Fournier et al. 2012), TMB allows for parameters to be fit in phases and we include the `multiphase` argument in `sdmTMB::sdmTMBcontrol()` to allow this. For high-dimensional models (many fixed and random effects), phased estimation may be faster, but this is not always the case because it requires an extra call to `TMB::MakeADFun()`. In sdmTMB, phased estimation proceeds by first estimating all fixed-effect parameters contributing to the likelihood (holding random effects constant at initial values). In the second phase, the random-effect parameters (and their variances) are also estimated. Fixed-effect parameters are also estimated in the second phase and are initialized at their estimates from the first phase.

In some cases, a single call to `stats::nlminb()` may not be result in convergence (e.g., the maximum gradient of the marginal likelihood with respect to fixed-effect parameters is not small enough yet), and the algorithm may need to be run multiple times. In the `sdmTMB::sdmTMBcontrol()` function, we include an argument `nlminb_loops` that will restart the optimization at the previous best values. The number of `nlminb_loops` should generally be small (e.g., 2 or 3), and defaults to 1. For some sdmTMB models, the Hessian may also be unstable and need to be re-evaluated. We do this optionally with the `stats::optimHess()` routine after the call to `stats::nlminb()`. The `stats::optimHess()` function implements a Newton optimization routine to find the Hessian, and we include the argument `newton_loops` in `sdmTMB::sdmTMBcontrol()` to allow for multiple function evaluations (each starting at the previous best value). By default, this is set to 1 evaluation. The updated parameters are only accepted if they result in a lower negative marginal log likelihood. If a model is already fit, the function `sdmTMB::run_extra_optimization()` can run additional optimization loops with either routine to further reduce the maximum gradient.

## Assessing convergence

Much of the guidance around diagnostics and glmmTMB also applies to sdmTMB, e.g. the glmmTMB vignette on troubleshooting. Optimization with `stats::nlminb()` involves specifying the number of iterations and evaluations (`eval.max` and `iter.max`) and the tolerances (`abs.tol`, `rel.tol`, `x.tol`, `xf.tol`)—a greater number of iterations and smaller tolerance thresholds increase the chance that the optimal solution is found, but more evaluations translates into longer computation time. Warnings of non-positive-definite Hessian matrices (accompanied by parameters with `NA`s for standard errors) often mean models are improperly specified given the data. Standard errors can be observed in the output of `print.sdmTMB()` or by checking `fit$sd_report`. The maximum gradient of the marginal likelihood with respect to fixed-effect parameters can be checked by inspecting (`fit$gradients`). Guidance varies, but gradients should be very close to zero (e.g., on the order of \(10^{-3}\) or smaller after reasonable parameter scaling) before assuming the fitting routine is consistent with convergence. If maximum gradients are already relatively small, they can sometimes be reduced further with additional optimization calls beginning at the previous best parameter vector as described above with `sdmTMB::run_extra_optimization()`.

# Notation reference tables

Tables of all major indices (Table 1) and symbols (Table 2) are provided here for quick reference.

Table 1: Subscript notation
Symbol | Description
--- | ---
\(\mathbf{s}\) | Index for space; a vector of x and y coordinates
\(t\) | Index for time
\(g\) | Group
\(p\) | Index for time-varying coefficient
\(l\) | Index for spatially varying coefficient (SVC)

Table 2: Symbol notation, code representation (in model output or in model template code), and descriptions.
Symbol | Code | Description
--- | --- | ---
\(y\) | `y_i` | Observed response data
\(\mu\) | `mu_i` | Mean
\(\eta\) | `eta_i` | Linear predictor before applying the inverse link (\(f^{-1}\))
\(\phi\) | `phi` | A dispersion parameter for a distribution
\(f\) | `fit$family$link` | Link function
\(f^{-1}\) | `fit$family$linkinv` | Inverse link function
\(\boldsymbol{\beta}\) | `b_j` | Parameter vector
\(\mathbf{X}\) | `X_ij` | A predictor model matrix
\(\mathbf{Z}\) | `Zt_list` | Random effect design matrix (list of sparse blocks; rows correspond to observations; subset by group \(g\))
\(O_{\mathbf{s}, t}\) | `offset` | An offset variable at point \(\mathbf{s}\) and time \(t\)
\(\omega_{\mathbf{s}}\) | `omega_s` | Spatial random field at point \(\mathbf{s}\) (knot)
\(\omega_{\mathbf{s}}^*\) | `omega_s_A` | Spatial random field at point \(\mathbf{s}\) (interpolated)
\(\zeta_{\mathbf{s}}\) | `zeta_s` | Spatially varying coefficient random field at point \(\mathbf{s}\) (knot)
\(\zeta_{\mathbf{s}}^*\) | `zeta_s_A` | Spatially varying coefficient random field at point \(\mathbf{s}\) (interpolated)
\(\epsilon_{\mathbf{s}, t}\) | `epsilon_st` | Spatiotemporal random field at point \(\mathbf{s}\) and time \(t\) (knot)
\(\epsilon_{\mathbf{s}, t}^*\) | `epsilon_st_A` | Spatiotemporal random field at point \(\mathbf{s}\) and time \(t\) (interpolated)
\(\delta_{\mathbf{s},t}\) | `b_t` | AR(1) or random walk spatiotemporal deviations (knot)
\(\alpha_g\) | `RE` | IID random intercept deviation for group \(g\)
\(\mathbf{b}_{g}\) | `re_b_pars` | Vector of random intercepts/slopes for group \(g\)
\(\boldsymbol{\Sigma}_\omega\) | `-` | Spatial random field covariance matrix
\(\boldsymbol{\Sigma}_\zeta\) | `-` | Spatially varying coefficient random field covariance matrix
\(\boldsymbol{\Sigma}_\epsilon\) | `-` | Spatiotemporal random field covariance matrix
\(\mathbf{Q}_\omega\) | `Q_s` | Spatial random field precision matrix
\(\mathbf{Q}_\zeta\) | `Q_s` | Spatially varying coefficient random field precision matrix
\(\mathbf{Q}_\epsilon\) | `Q_st` | Spatiotemporal random field precision matrix
\(\boldsymbol{\Sigma}_{g}\) | `re_cov_pars` | Random effect covariance per grouping factor (Cholesky SD + correlation params)
\(\sigma_\alpha^2\) | `sigma_G` | IID random intercept variance
\(\sigma_\epsilon^2\) | `sigma_E` | Spatiotemporal random field marginal variance
\(\sigma_\omega^2\) | `sigma_O` | Spatial random field marginal variance
\(\sigma_\zeta^2\) | `sigma_Z` | Spatially varying coefficient random field marginal variance
\(\kappa_\omega\) | `kappa(0)` | Spatial decorrelation rate
\(\kappa_\epsilon\) | `kappa(1)` | Spatiotemporal decorrelation rate
\(\rho\) | `ar1_rho` | Correlation between random fields in subsequent time steps
\(\rho_{\gamma}\) | `rho_time` | Correlation between time-varying coefficients in subsequent time steps
\(Q^{\mathrm{gg}}\) | `gengamma_Q` | Generalized gamma shape parameter (\(Q \rightarrow 0\) gives lognormal; \(Q=\phi\) gives gamma)
\(\mathbf{A}\) | `A` | Sparse projection matrix to interpolate between knot and data locations
\(\mathbf{H}\) | `H` | 2-parameter rotation matrix used to define anisotropy

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