RW {mvgam}

Specify autoregressive dynamic processes

Description

Set up autoregressive or autoregressive moving average trend models in mvgam. These functions do not evaluate their arguments – they exist purely to help set up a model with particular autoregressive trend models.

Usage

RW(ma = FALSE, cor = FALSE)

AR(p = 1, ma = FALSE, cor = FALSE)

CAR(p = 1)

VAR(ma = FALSE, cor = FALSE)

Arguments

ma

Logical Include moving average terms of order 1? Default is FALSE.

cor

Logical Include correlated process errors as part of a multivariate normal process model? If TRUE and if n_series > 1 in the supplied data, a fully structured covariance matrix will be estimated for the process errors. Default is FALSE.

p

A non-negative integer specifying the autoregressive (AR) order. Default is 1. Cannot currently be larger than 3 for AR terms, and cannot be anything other than 1 for continuous time AR (CAR) terms

Value

An object of class mvgam_trend, which contains a list of arguments to be interpreted by the parsing functions in mvgam

Examples

## Not run: 
# A short example to illustrate CAR(1) models
# Function to simulate CAR1 data with seasonality
sim_corcar1 = function(n = 120,
                      phi = 0.5,
                      sigma = 1,
                      sigma_obs = 0.75){
# Sample irregularly spaced time intervals
time_dis <- c(0, runif(n - 1, -0.1, 1))
time_dis[time_dis < 0] <- 0; time_dis <- time_dis * 5

# Set up the latent dynamic process
x <- vector(length = n); x[1] <- -0.3
for(i in 2:n){
 # zero-distances will cause problems in sampling, so mvgam uses a
 # minimum threshold; this simulation function emulates that process
 if(time_dis[i] == 0){
   x[i] <- rnorm(1, mean = (phi ^ 1e-12) * x[i - 1], sd = sigma)
  } else {
    x[i] <- rnorm(1, mean = (phi ^ time_dis[i]) * x[i - 1], sd = sigma)
  }
}

# Add 12-month seasonality
cov1 <- sin(2 * pi * (1 : n) / 12); cov2 <- cos(2 * pi * (1 : n) / 12)
beta1 <- runif(1, 0.3, 0.7); beta2 <- runif(1, 0.2, 0.5)
seasonality <- beta1 * cov1 + beta2 * cov2

# Take Gaussian observations with error and return
data.frame(y = rnorm(n, mean = x + seasonality, sd = sigma_obs),
           season = rep(1:12, 20)[1:n],
           time = cumsum(time_dis))
}

# Sample two time series
dat <- rbind(dplyr::bind_cols(sim_corcar1(phi = 0.65,
                                         sigma_obs = 0.55),
                             data.frame(series = 'series1')),
            dplyr::bind_cols(sim_corcar1(phi = 0.8,
                             sigma_obs = 0.35),
                             data.frame(series = 'series2'))) %>%
      dplyr::mutate(series = as.factor(series))

# mvgam with CAR(1) trends and series-level seasonal smooths; the
# State-Space representation (using trend_formula) will be more efficient
mod <- mvgam(formula = y ~ 1,
            trend_formula = ~ s(season, bs = 'cc',
                                k = 5, by = trend),
            trend_model = CAR(),
            data = dat,
            family = gaussian(),
            samples = 300,
            chains = 2)

# View usual summaries and plots
summary(mod)
conditional_effects(mod, type = 'expected')
plot(mod, type = 'trend', series = 1)
plot(mod, type = 'trend', series = 2)
plot(mod, type = 'residuals', series = 1)
plot(mod, type = 'residuals', series = 2)

## End(Not run)