Optional priors/penalties on model parameters. This results in penalized likelihood within TMB or can be used as priors if the model is passed to tmbstan (see the Bayesian vignette).

**Note that Jacobian adjustments are only made if bayesian = TRUE** when the

`sdmTMB()`

model is fit. I.e., the final model will be fit with tmbstan
and priors are specified then `bayesian`

should be set to `TRUE`

. Otherwise,
leave `bayesian = FALSE`

.`pc_matern()`

is the Penalized Complexity prior for the Matern
covariance function.

## Usage

```
sdmTMBpriors(
matern_s = pc_matern(range_gt = NA, sigma_lt = NA),
matern_st = pc_matern(range_gt = NA, sigma_lt = NA),
phi = halfnormal(NA, NA),
ar1_rho = normal(NA, NA),
tweedie_p = normal(NA, NA),
b = normal(NA, NA)
)
normal(location = 0, scale = 1)
halfnormal(location = 0, scale = 1)
mvnormal(location = 0, scale = diag(length(location)))
pc_matern(range_gt, sigma_lt, range_prob = 0.05, sigma_prob = 0.05)
```

## Arguments

- matern_s
A PC (Penalized Complexity) prior (

`pc_matern()`

) on the spatial random field Matérn parameters.- matern_st
Same as

`matern_s`

but for the spatiotemporal random field. Note that you will likely want to set`share_fields = FALSE`

if you choose to set both a spatial and spatiotemporal Matérn PC prior since they both include a prior on the spatial range parameter.- phi
A

`halfnormal()`

prior for the dispersion parameter in the observation distribution.- ar1_rho
A

`normal()`

prior for the AR1 random field parameter. Note the parameter has support`-1 < ar1_rho < 1`

.- tweedie_p
A

`normal()`

prior for the Tweedie power parameter. Note the parameter has support`1 < tweedie_p < 2`

so choose a mean appropriately.- b
`normal()`

priors for the main population-level 'beta' effects.- location
Location parameter(s).

- scale
Scale parameter. For

`normal()`

/`halfnormal()`

: standard deviation(s). For`mvnormal()`

: variance-covariance matrix.- range_gt
A value one expects the spatial or spatiotemporal range is

**g**reater**t**han with`1 - range_prob`

probability.- sigma_lt
A value one expects the spatial or spatiotemporal marginal standard deviation (

`sigma_O`

or`sigma_E`

internally) is**l**ess**t**han with`1 - sigma_prob`

probability.- range_prob
Probability. See description for

`range_gt`

.- sigma_prob
Probability. See description for

`sigma_lt`

.

## Details

Meant to be passed to the `priors`

argument in `sdmTMB()`

.

`normal()`

and `halfnormal()`

define normal and half-normal priors that, at
this point, must have a location (mean) parameter of 0. `halfnormal()`

is the
same as `normal()`

but can be used to make the syntax clearer. It is intended
to be used for parameters that have support `> 0`

.

See https://arxiv.org/abs/1503.00256 for a description of the
PC prior for Gaussian random fields. Quoting the discussion (and substituting
the argument names in `pc_matern()`

):
"In the simulation study we observe good coverage of the equal-tailed 95%
credible intervals when the prior satisfies `P(sigma > sigma_lt) = 0.05`

and
`P(range < range_gt) = 0.05`

, where `sigma_lt`

is between 2.5 to 40 times
the true marginal standard deviation and `range_gt`

is between 1/10 and 1/2.5
of the true range." Also see `INLA::inla.spde2.pcmatern()`

.

Keep in mind that the range is dependent on the units and scale of the coordinate system. In practice, you may choose to try fitting the model without a PC prior and then constraining the model from there. A better option would be to simulate from a model with a given range and sigma to choose reasonable values for the system or base the prior on knowledge from a model fit to a similar system but with more spatial information in the data.

## References

Fuglstad, G.-A., Simpson, D., Lindgren, F., and Rue, H. (2016) Constructing Priors that Penalize the Complexity of Gaussian Random Fields. arXiv:1503.00256

Simpson, D., Rue, H., Martins, T., Riebler, A., and Sørbye, S. (2015) Penalising model component complexity: A principled, practical approach to constructing priors. arXiv:1403.4630

## Examples

```
normal(0, 1)
#> [,1] [,2]
#> [1,] 0 1
#> attr(,"dist")
#> [1] "normal"
halfnormal(0, 1)
#> [,1] [,2]
#> [1,] 0 1
#> attr(,"dist")
#> [1] "normal"
mvnormal(c(0, 0))
#> [,1] [,2] [,3]
#> [1,] 0 1 0
#> [2,] 0 0 1
#> attr(,"dist")
#> [1] "mvnormal"
pc_matern(range_gt = 5, sigma_lt = 1)
#> [1] 5.00 1.00 0.05 0.05
#> attr(,"dist")
#> [1] "pc_matern"
plot_pc_matern(range_gt = 5, sigma_lt = 1)
if (inla_installed()) {
d <- subset(pcod, year > 2011)
pcod_spde <- make_mesh(d, c("X", "Y"), cutoff = 30)
# - no priors on population-level effects (`b`)
# - halfnormal(0, 10) prior on dispersion parameter `phi`
# - Matern PC priors on spatial `matern_s` and spatiotemporal
# `matern_st` random field parameters
m <- sdmTMB(density ~ s(depth, k = 3),
data = d, mesh = pcod_spde, family = tweedie(),
share_range = FALSE, time = "year",
priors = sdmTMBpriors(
phi = halfnormal(0, 10),
matern_s = pc_matern(range_gt = 5, sigma_lt = 1),
matern_st = pc_matern(range_gt = 5, sigma_lt = 1)
)
)
# - no prior on intercept
# - normal(0, 1) prior on depth coefficient
# - no prior on the dispersion parameter `phi`
# - Matern PC prior
m <- sdmTMB(density ~ depth_scaled,
data = d, mesh = pcod_spde, family = tweedie(),
spatiotemporal = "off",
priors = sdmTMBpriors(
b = normal(c(NA, 0), c(NA, 1)),
matern_s = pc_matern(range_gt = 5, sigma_lt = 1)
)
)
# You get a prior, you get a prior, you get a prior!
# (except on the annual means; see the `NA`s)
m <- sdmTMB(density ~ 0 + depth_scaled + depth_scaled2 + as.factor(year),
data = d, time = "year", mesh = pcod_spde, family = tweedie(link = "log"),
share_range = FALSE, spatiotemporal = "AR1",
priors = sdmTMBpriors(
b = normal(c(0, 0, NA, NA, NA), c(2, 2, NA, NA, NA)),
phi = halfnormal(0, 10),
tweedie_p = normal(1.5, 2),
ar1_rho = normal(0, 1),
matern_s = pc_matern(range_gt = 5, sigma_lt = 1),
matern_st = pc_matern(range_gt = 5, sigma_lt = 1))
)
}
```