The purpose of this vignette is to show how the `mvgam`

package can be used to estimate and forecast regression coefficients
that vary through time.

Dynamic fixed-effect coefficients (often referred to as dynamic
linear models) can be readily incorporated into GAMs / DGAMs. In
`mvgam`

, the `dynamic()`

formula wrapper offers a
convenient interface to set these up. The plan is to incorporate a range
of dynamic options (such as random walk, AR1 etc…) but for the moment
only low-rank Gaussian Process (GP) smooths are allowed (making use
either of the `gp`

basis in `mgcv`

of of Hilbert
space approximate GPs). These are advantageous over splines or random
walk effects for several reasons. First, GPs will force the time-varying
effect to be smooth. This often makes sense in reality, where we would
not expect a regression coefficient to change rapidly from one time
point to the next. Second, GPs provide information on the ‘global’
dynamics of a time-varying effect through their length-scale parameters.
This means we can use them to provide accurate forecasts of how an
effect is expected to change in the future, something that we couldn’t
do well if we used splines to estimate the effect. An example below
illustrates.

Simulate a time-varying coefficient using a squared exponential
Gaussian Process function with length scale \(\rho\)=10. We will do this using an
internal function from `mvgam`

(the `sim_gp`

function):

```
set.seed(1111)
N <- 200
beta_temp <- mvgam:::sim_gp(rnorm(1),
alpha_gp = 0.75,
rho_gp = 10,
h = N) + 0.5
```

A plot of the time-varying coefficient shows that it changes smoothly through time:

```
plot(beta_temp, type = 'l', lwd = 3,
bty = 'l', xlab = 'Time', ylab = 'Coefficient',
col = 'darkred')
box(bty = 'l', lwd = 2)
```

Next we need to simulate the values of the covariate, which we will
call `temp`

(to represent \(temperature\)). In this case we just use a
standard normal distribution to simulate this covariate:

Finally, simulate the outcome variable, which is a Gaussian observation process (with observation error) over the time-varying effect of \(temperature\)

```
out <- rnorm(N, mean = 4 + beta_temp * temp,
sd = 0.25)
time <- seq_along(temp)
plot(out, type = 'l', lwd = 3,
bty = 'l', xlab = 'Time', ylab = 'Outcome',
col = 'darkred')
box(bty = 'l', lwd = 2)
```

Gather the data into a `data.frame`

for fitting models,
and split the data into training and testing folds.

`dynamic()`

functionTime-varying coefficients can be fairly easily set up using the
`s()`

or `gp()`

wrapper functions in
`mvgam`

formulae by fitting a nonlinear effect of
`time`

and using the covariate of interest as the numeric
`by`

variable (see `?mgcv::s`

or
`?brms::gp`

for more details). The `dynamic()`

formula wrapper offers a way to automate this process, and will
eventually allow for a broader variety of time-varying effects (such as
random walk or AR processes). Depending on the arguments that are
specified to `dynamic`

, it will either set up a low-rank GP
smooth function using `s()`

with `bs = 'gp'`

and a
fixed value of the length scale parameter \(\rho\), or it will set up a Hilbert space
approximate GP using the `gp()`

function with
`c=5/4`

so that \(\rho\) is
estimated (see `?dynamic`

for more details). In this first
example we will use the `s()`

option, and will mis-specify
the \(\rho\) parameter here as, in
practice, it is never known. This call to `dynamic()`

will
set up the following smooth:
`s(time, by = temp, bs = "gp", m = c(-2, 8, 2), k = 40)`

```
mod <- mvgam(out ~ dynamic(temp, rho = 8, stationary = TRUE, k = 40),
family = gaussian(),
data = data_train,
silent = 2)
```

Inspect the model summary, which shows how the `dynamic()`

wrapper was used to construct a low-rank Gaussian Process smooth
function:

```
summary(mod, include_betas = FALSE)
#> GAM formula:
#> out ~ s(time, by = temp, bs = "gp", m = c(-2, 8, 2), k = 40)
#> <environment: 0x0000026c107b3fd8>
#>
#> Family:
#> gaussian
#>
#> Link function:
#> identity
#>
#> Trend model:
#> None
#>
#> N series:
#> 1
#>
#> N timepoints:
#> 190
#>
#> Status:
#> Fitted using Stan
#> 4 chains, each with iter = 1000; warmup = 500; thin = 1
#> Total post-warmup draws = 2000
#>
#>
#> Observation error parameter estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> sigma_obs[1] 0.23 0.25 0.28 1 1709
#>
#> GAM coefficient (beta) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> (Intercept) 4 4 4.1 1 1960
#>
#> Approximate significance of GAM smooths:
#> edf Ref.df Chi.sq p-value
#> s(time):temp 17 40 164 <2e-16 ***
#> ---
#> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#>
#> Stan MCMC diagnostics:
#> n_eff / iter looks reasonable for all parameters
#> Rhat looks reasonable for all parameters
#> 0 of 2000 iterations ended with a divergence (0%)
#> 0 of 2000 iterations saturated the maximum tree depth of 12 (0%)
#> E-FMI indicated no pathological behavior
#>
#> Samples were drawn using NUTS(diag_e) at Wed Sep 04 11:52:26 AM 2024.
#> For each parameter, n_eff is a crude measure of effective sample size,
#> and Rhat is the potential scale reduction factor on split MCMC chains
#> (at convergence, Rhat = 1)
```

Because this model used a spline with a `gp`

basis, it’s
smooths can be visualised just like any other `gam`

. We can
plot the estimates for the in-sample and out-of-sample periods to see
how the Gaussian Process function produces sensible smooth forecasts.
Here we supply the full dataset to the `newdata`

argument in
`plot_mvgam_smooth`

to inspect posterior forecasts of the
time-varying smooth function. Overlay the true simulated function to see
that the model has adequately estimated it’s dynamics in both the
training and testing data partitions

```
plot_mvgam_smooth(mod, smooth = 1, newdata = data)
abline(v = 190, lty = 'dashed', lwd = 2)
lines(beta_temp, lwd = 2.5, col = 'white')
lines(beta_temp, lwd = 2)
```

We can also use `plot_predictions`

from the
`marginaleffects`

package to visualise the time-varying
coefficient for what the effect would be estimated to be at different
values of \(temperature\):

```
require(marginaleffects)
range_round = function(x){
round(range(x, na.rm = TRUE), 2)
}
plot_predictions(mod,
newdata = datagrid(time = unique,
temp = range_round),
by = c('time', 'temp', 'temp'),
type = 'link')
```

This results in sensible forecasts of the observations as well

The syntax is very similar if we wish to estimate the parameters of
the underlying Gaussian Process, this time using a Hilbert space
approximation. We simply omit the `rho`

argument in
`dynamic`

to make this happen. This will set up a call
similar to `gp(time, by = 'temp', c = 5/4, k = 40)`

.

This model summary now contains estimates for the marginal deviation and length scale parameters of the underlying Gaussian Process function:

```
summary(mod, include_betas = FALSE)
#> GAM formula:
#> out ~ gp(time, by = temp, c = 5/4, k = 40, scale = TRUE)
#> <environment: 0x0000026c107b3fd8>
#>
#> Family:
#> gaussian
#>
#> Link function:
#> identity
#>
#> Trend model:
#> None
#>
#> N series:
#> 1
#>
#> N timepoints:
#> 190
#>
#> Status:
#> Fitted using Stan
#> 4 chains, each with iter = 1000; warmup = 500; thin = 1
#> Total post-warmup draws = 2000
#>
#>
#> Observation error parameter estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> sigma_obs[1] 0.24 0.26 0.3 1 2285
#>
#> GAM coefficient (beta) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> (Intercept) 4 4 4.1 1 3056
#>
#> GAM gp term marginal deviation (alpha) and length scale (rho) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> alpha_gp(time):temp 0.630 0.880 1.400 1.00 619
#> rho_gp(time):temp 0.026 0.053 0.069 1.01 487
#>
#> Stan MCMC diagnostics:
#> n_eff / iter looks reasonable for all parameters
#> Rhat looks reasonable for all parameters
#> 0 of 2000 iterations ended with a divergence (0%)
#> 0 of 2000 iterations saturated the maximum tree depth of 12 (0%)
#> E-FMI indicated no pathological behavior
#>
#> Samples were drawn using NUTS(diag_e) at Wed Sep 04 11:53:17 AM 2024.
#> For each parameter, n_eff is a crude measure of effective sample size,
#> and Rhat is the potential scale reduction factor on split MCMC chains
#> (at convergence, Rhat = 1)
```

Effects for `gp()`

terms can also be plotted as
smooths:

Here we will use openly available data on marine survival of Chinook
salmon to illustrate how time-varying effects can be used to improve
ecological time series models. Scheuerell
and Williams (2005) used a dynamic linear model to examine the
relationship between marine survival of Chinook salmon and an index of
ocean upwelling strength along the west coast of the USA. The authors
hypothesized that stronger upwelling in April should create better
growing conditions for phytoplankton, which would then translate into
more zooplankton and provide better foraging opportunities for juvenile
salmon entering the ocean. The data on survival is measured as a
proportional variable over 42 years (1964–2005) and is available in the
`MARSS`

package:

```
load(url('https://github.com/atsa-es/MARSS/raw/master/data/SalmonSurvCUI.rda'))
dplyr::glimpse(SalmonSurvCUI)
#> Rows: 42
#> Columns: 3
#> $ year <int> 1964, 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 19…
#> $ logit.s <dbl> -3.46, -3.32, -3.58, -3.03, -3.61, -3.35, -3.93, -4.19, -4.82,…
#> $ CUI.apr <int> 57, 5, 43, 11, 47, -21, 25, -2, -1, 43, 2, 35, 0, 1, -1, 6, -7…
```

First we need to prepare the data for modelling. The variable
`CUI.apr`

will be standardized to make it easier for the
sampler to estimate underlying GP parameters for the time-varying
effect. We also need to convert the survival back to a proportion, as in
its current form it has been logit-transformed (this is because most
time series packages cannot handle proportional data). As usual, we also
need to create a `time`

indicator and a `series`

indicator for working in `mvgam`

:

```
SalmonSurvCUI %>%
# create a time variable
dplyr::mutate(time = dplyr::row_number()) %>%
# create a series variable
dplyr::mutate(series = as.factor('salmon')) %>%
# z-score the covariate CUI.apr
dplyr::mutate(CUI.apr = as.vector(scale(CUI.apr))) %>%
# convert logit-transformed survival back to proportional
dplyr::mutate(survival = plogis(logit.s)) -> model_data
```

Inspect the data

```
dplyr::glimpse(model_data)
#> Rows: 42
#> Columns: 6
#> $ year <int> 1964, 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1…
#> $ logit.s <dbl> -3.46, -3.32, -3.58, -3.03, -3.61, -3.35, -3.93, -4.19, -4.82…
#> $ CUI.apr <dbl> 2.37949804, 0.03330223, 1.74782994, 0.30401713, 1.92830654, -…
#> $ time <int> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18…
#> $ series <fct> salmon, salmon, salmon, salmon, salmon, salmon, salmon, salmo…
#> $ survival <dbl> 0.030472033, 0.034891409, 0.027119717, 0.046088827, 0.0263393…
```

Plot features of the outcome variable, which shows that it is a proportional variable with particular restrictions that we want to model:

`mvgam`

can easily handle data that are bounded at 0 and 1
with a Beta observation model (using the `mgcv`

function
`betar()`

, see `?mgcv::betar`

for details). First
we will fit a simple State-Space model that uses an AR1 dynamic process
model with no predictors and a Beta observation model:

```
mod0 <- mvgam(formula = survival ~ 1,
trend_model = AR(),
noncentred = TRUE,
family = betar(),
data = model_data,
silent = 2)
```

The summary of this model shows good behaviour of the Hamiltonian Monte Carlo sampler and provides useful summaries on the Beta observation model parameters:

```
summary(mod0)
#> GAM formula:
#> survival ~ 1
#> <environment: 0x0000026c107b3fd8>
#>
#> Family:
#> beta
#>
#> Link function:
#> logit
#>
#> Trend model:
#> AR()
#>
#> N series:
#> 1
#>
#> N timepoints:
#> 42
#>
#> Status:
#> Fitted using Stan
#> 4 chains, each with iter = 1000; warmup = 500; thin = 1
#> Total post-warmup draws = 2000
#>
#>
#> Observation precision parameter estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> phi[1] 98 270 650 1.01 272
#>
#> GAM coefficient (beta) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> (Intercept) -4.7 -4.4 -4.1 1 570
#>
#> Latent trend parameter AR estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> ar1[1] 0.037 0.69 0.98 1.00 570
#> sigma[1] 0.120 0.45 0.73 1.01 225
#>
#> Stan MCMC diagnostics:
#> n_eff / iter looks reasonable for all parameters
#> Rhat looks reasonable for all parameters
#> 1 of 2000 iterations ended with a divergence (0.05%)
#> *Try running with larger adapt_delta to remove the divergences
#> 0 of 2000 iterations saturated the maximum tree depth of 12 (0%)
#> E-FMI indicated no pathological behavior
#>
#> Samples were drawn using NUTS(diag_e) at Wed Sep 04 11:54:12 AM 2024.
#> For each parameter, n_eff is a crude measure of effective sample size,
#> and Rhat is the potential scale reduction factor on split MCMC chains
#> (at convergence, Rhat = 1)
```

A plot of the underlying dynamic component shows how it has easily handled the temporal evolution of the time series:

Now we can increase the complexity of our model by constructing and
fitting a State-Space model with a time-varying effect of the coastal
upwelling index in addition to the autoregressive dynamics. We again use
a Beta observation model to capture the restrictions of our proportional
observations, but this time will include a `dynamic()`

effect
of `CUI.apr`

in the latent process model. We do not specify
the \(\rho\) parameter, instead opting
to estimate it using a Hilbert space approximate GP:

```
mod1 <- mvgam(formula = survival ~ 1,
trend_formula = ~ dynamic(CUI.apr, k = 25, scale = FALSE),
trend_model = AR(),
noncentred = TRUE,
family = betar(),
data = model_data,
silent = 2)
```

The summary for this model now includes estimates for the time-varying GP parameters:

```
summary(mod1, include_betas = FALSE)
#> GAM observation formula:
#> survival ~ 1
#> <environment: 0x0000026c107b3fd8>
#>
#> GAM process formula:
#> ~dynamic(CUI.apr, k = 25, scale = FALSE)
#> <environment: 0x0000026c107b3fd8>
#>
#> Family:
#> beta
#>
#> Link function:
#> logit
#>
#> Trend model:
#> AR()
#>
#> N process models:
#> 1
#>
#> N series:
#> 1
#>
#> N timepoints:
#> 42
#>
#> Status:
#> Fitted using Stan
#> 4 chains, each with iter = 1000; warmup = 500; thin = 1
#> Total post-warmup draws = 2000
#>
#>
#> Observation precision parameter estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> phi[1] 180 360 670 1.01 504
#>
#> GAM observation model coefficient (beta) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> (Intercept) -6.1 -2.4 1.8 1 1605
#>
#> Process model AR parameter estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> ar1[1] 0.48 0.89 1 1.01 681
#>
#> Process error parameter estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> sigma[1] 0.18 0.35 0.57 1.02 488
#>
#> GAM process model coefficient (beta) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> (Intercept)_trend -5.8 -1.5 2.3 1 1670
#>
#> GAM process model gp term marginal deviation (alpha) and length scale (rho) estimates:
#> 2.5% 50% 97.5% Rhat n_eff
#> alpha_gp_time_byCUI_apr_trend 0.03 0.32 1.3 1 629
#> rho_gp_time_byCUI_apr_trend 1.40 5.70 32.0 1 560
#>
#> Stan MCMC diagnostics:
#> n_eff / iter looks reasonable for all parameters
#> Rhat looks reasonable for all parameters
#> 114 of 2000 iterations ended with a divergence (5.7%)
#> *Try running with larger adapt_delta to remove the divergences
#> 0 of 2000 iterations saturated the maximum tree depth of 12 (0%)
#> E-FMI indicated no pathological behavior
#>
#> Samples were drawn using NUTS(diag_e) at Wed Sep 04 11:55:32 AM 2024.
#> For each parameter, n_eff is a crude measure of effective sample size,
#> and Rhat is the potential scale reduction factor on split MCMC chains
#> (at convergence, Rhat = 1)
```

The estimates for the underlying dynamic process, and for the hindcasts, haven’t changed much:

But the process error parameter \(\sigma\) is slightly smaller for this model than for the first model:

```
# Extract estimates of the process error 'sigma' for each model
mod0_sigma <- as.data.frame(mod0, variable = 'sigma', regex = TRUE) %>%
dplyr::mutate(model = 'Mod0')
mod1_sigma <- as.data.frame(mod1, variable = 'sigma', regex = TRUE) %>%
dplyr::mutate(model = 'Mod1')
sigmas <- rbind(mod0_sigma, mod1_sigma)
# Plot using ggplot2
require(ggplot2)
ggplot(sigmas, aes(y = `sigma[1]`, fill = model)) +
geom_density(alpha = 0.3, colour = NA) +
coord_flip()
```

Why does the process error not need to be as flexible in the second
model? Because the estimates of this dynamic process are now informed
partly by the time-varying effect of upwelling, which we can visualise
on the link scale using `plot()`

:

A key question when fitting multiple time series models is whether
one of them provides better predictions than the other. There are
several options in `mvgam`

for exploring this quantitatively.
First, we can compare models based on in-sample approximate
leave-one-out cross-validation as implemented in the popular
`loo`

package:

The second model has the larger Expected Log Predictive Density
(ELPD), meaning that it is slightly favoured over the simpler model that
did not include the time-varying upwelling effect. However, the two
models certainly do not differ by much. But this metric only compares
in-sample performance, and we are hoping to use our models to produce
reasonable forecasts. Luckily, `mvgam`

also has routines for
comparing models using approximate leave-future-out cross-validation.
Here we refit both models to a reduced training set (starting at time
point 30) and produce approximate 1-step ahead forecasts. These
forecasts are used to estimate forecast ELPD before expanding the
training set one time point at a time. We use Pareto-smoothed importance
sampling to reweight posterior predictions, acting as a kind of particle
filter so that we don’t need to refit the model too often (you can read
more about how this process works in Bürkner et al. 2020).

The model with the time-varying upwelling effect tends to provides better 1-step ahead forecasts, with a higher total forecast ELPD

We can also plot the ELPDs for each model as a contrast. Here, values less than zero suggest the time-varying predictor model (Mod1) gives better 1-step ahead forecasts:

```
plot(x = 1:length(lfo_mod0$elpds) + 30,
y = lfo_mod0$elpds - lfo_mod1$elpds,
ylab = 'ELPDmod0 - ELPDmod1',
xlab = 'Evaluation time point',
pch = 16,
col = 'darkred',
bty = 'l')
abline(h = 0, lty = 'dashed')
```

A useful exercise to further expand this model would be to think
about what kinds of predictors might impact measurement error, which
could easily be implemented into the observation formula in
`mvgam()`

. But for now, we will leave the model as-is.

The following papers and resources offer a lot of useful material about dynamic linear models and how they can be applied / evaluated in practice:

Bürkner, PC, Gabry, J and Vehtari, A Approximate
leave-future-out cross-validation for Bayesian time series models.
*Journal of Statistical Computation and Simulation*. 90:14 (2020)
2499-2523.

Herrero, Asier, et al. From
the individual to the landscape and back: time‐varying effects of
climate and herbivory on tree sapling growth at distribution limits.
*Journal of Ecology* 104.2 (2016): 430-442.

Holmes, Elizabeth E., Eric J. Ward, and Wills Kellie. “MARSS:
multivariate autoregressive state-space models for analyzing time-series
data.” *R Journal*. 4.1 (2012): 11.

Scheuerell, Mark D., and John G. Williams. Forecasting
climate induced changes in the survival of Snake River Spring/Summer
Chinook Salmon (*Oncorhynchus Tshawytscha*) *Fisheries
Oceanography* 14 (2005): 448–57.

I’m actively seeking PhD students and other researchers to work in
the areas of ecological forecasting, multivariate model evaluation and
development of `mvgam`

. Please reach out if you are
interested (n.clark’at’uq.edu.au)