# 8 State-space models

#### Jan-Ole Fischer

> Before diving into this vignette, we recommend reading the vignettes Introduction to LaMa and Automatic differentiation via RTMB.

    library(LaMa)

This vignette shows how to fit state-space models which can be interpreted as generalisation of HMMs to continuous state spaces. Several approaches exist to fitting such models, but Langrock (2011) showed that very general state-space models can be fitted via approximate maximum likelihood estimation, when the continuous state space is finely discretised. This is equivalent to numerical integration over the state process using midpoint quadrature.

Here, we will showcase this approach for a basic stochastic volatility model which, while being very simple, captures most of the stylised facts of financial time series. In this model the unobserved marked volatility is described by an AR(1) process:

\[ g_t = \phi g_{t-1} + \sigma \eta_t, \qquad \eta_t \sim N(0,1), \] with autoregression parameter \(\phi < 1\) and dispersion parameter \(\sigma\). Share returns \(y_t\) can then be modelled as \[ y_t = \beta \epsilon_t \exp(g_t / 2), \] where \(\epsilon_t \sim N(0,1)\) and \(\beta > 0\) is the baseline standard deviation of the returns (when \(g_t\) is in equilibrium), which implies \[ y_t \mid g_t \sim N(0, (\beta e^{g_t / 2})^2). \]

## Example: stochastic volatility model

### Simulating data

We start by simulating data from the above specified model:

    beta = 2 # baseline standard deviation
    phi = 0.95 # AR parameter of the log-volatility process
    sigma = 0.5 # variability of the log-volatility process

    n = 1000
    set.seed(123)
    g = rep(NA, n)
    g[1] = rnorm(1, 0, sigma / sqrt(1-phi^2)) # stationary distribution of AR(1) process
    for(t in 2:n){
      # sampling next state based on previous state and AR(1) equation
      g[t] = rnorm(1, phi*g[t-1], sigma)
    }
    # sampling zero-mean observations with standard deviation given by latent process
    y = rnorm(n, 0, beta * exp(g/2))

    # share returns
    color = LaMaColors(2)
    oldpar = par(mar = c(5,4,3,4.5)+0.1)
    plot(y, type = "l", bty = "n", ylim = c(-40,20), yaxt = "n")
    # true underlying standard deviation
    lines(beta*exp(g)/7 - 40, col = color[2], lwd = 2)
    axis(side=2, at = seq(-20,20,by=5), labels = seq(-20,20,by=5))
    axis(side=4, at = seq(0,150,by=75)/7-40, labels = seq(0,150,by=75))
    mtext("standard deviation", side=4, line=3, at = -30)

    par(oldpar)

### Writing the negative log-likelihood function

To calculate the likelihood, we need to integrate over the state space. We approximate this high-dimensional integral using a midpoint quadrature which ultimately results in a fine discretisation of the continuous state space into the intervals `b` with width `h` and midpoints `bstar` (Langrock 2011). Thus, the likelihood below corresponds to a basic HMM likelihood with a large number of states.

    nll = function(par) {
      getAll(par, dat)
      phi   = plogis(logit_phi); REPORT(phi)
      sigma = exp(log_sigma);    REPORT(sigma)
      beta  = exp(log_beta);     REPORT(beta)
      b     = seq(-bm, bm, length = m + 1) # intervals for midpoint quadrature
      h     = b[2] - b[1]                  # interval width
      bstar = (b[-1] + b[-(m+1)]) / 2     # interval midpoints
      # approximating t.p.m. via midpoint quadrature
      Gamma = sapply(bstar, dnorm, mean = phi * bstar, sd = sigma) * h
      # stationary distribution of the AR(1) process: g ~ N(0, sigma^2/(1-phi^2))
      delta = h * dnorm(bstar, 0, sigma / sqrt(1 - phi^2))
      # state-dependent densities: y_t | g_t ~ N(0, (beta * exp(g_t/2))^2)
      allprobs = t(sapply(y, dnorm, mean = 0, sd = beta * exp(bstar / 2)))
      -forward(delta, Gamma, allprobs)
    }

### Fitting an SSM to the data

Because the transition matrix \(\Gamma\) is \(100 \times 100\) and dense, we set `TapeConfig(matmul = "atomic")` before building the AD objective.

By default (`"plain"`), `RTMB` unwraps every matrix multiplication into all its individual scalar operations on the tape. This is efficient when matrices are sparse, since the tape can exploit the structure. For large, dense matrices it produces an enormous tape — one scalar entry per element of the result — which slows both tape construction and subsequent gradient evaluation.

Declaring matrix multiplications as atomic treats each one as a single operation on the tape, keeping it compact regardless of matrix size. If your transition matrix were sparse — for example in models with local state dependence — the default `"plain"` setting would be preferable.

    TapeConfig(matmul = "atomic")

    bm = 5   # truncation of state space at ±bm
    m  = 100 # number of quadrature points

    par = list(
      logit_phi = qlogis(0.95),
      log_sigma = log(0.3),
      log_beta  = log(1)
    )
    dat = list(y = y, bm = bm, m = m)

    obj = MakeADFun(nll, par, silent = TRUE)
    system.time(
      opt <- nlminb(obj$par, obj$fn, obj$gr)
    )
    #>    user  system elapsed
    #>   0.388   0.004   0.395
    mod = report(obj)

### Results

    mod$phi    # true: 0.95
    #> [1] 0.951655
    mod$sigma  # true: 0.5
    #> [1] 0.4436881
    mod$beta   # true: 2
    #> [1] 2.18407

    ## decoding states
    b     = seq(-bm, bm, length = m + 1)
    h     = b[2] - b[1]
    bstar = (b[-1] + b[-(m+1)]) / 2
    Gamma    = sapply(bstar, dnorm, mean = mod$phi * bstar, sd = mod$sigma) * h
    delta    = h * dnorm(bstar, 0, mod$sigma / sqrt(1 - mod$phi^2))
    allprobs = t(sapply(y, dnorm, mean = 0, sd = mod$beta * exp(bstar / 2)))

    probs  = stateprobs(delta, Gamma, allprobs) # local/soft decoding
    states = viterbi(delta, Gamma, allprobs)    # global/hard decoding

    oldpar = par(mar = c(5,4,3,4.5)+0.1)
    plot(y, type = "l", bty = "n", ylim = c(-50,20), yaxt = "n")
    # with many states, plotting the full conditional distribution is more informative
    # than just the single most-probable state
    maxprobs = apply(probs, 1, max)
    for(t in 1:nrow(probs)){
      colend = round((probs[t,] / (maxprobs[t] * 5)) * 100)
      colend[which(colend < 10)] = paste0("0", colend[which(colend < 10)])
      points(rep(t, m), bstar * 4 - 35, col = paste0("#FFA200", colend), pch = 20)
    }
    # Viterbi-decoded path as a summary line
    lines(bstar[states] * 4 - 35)

    axis(side = 2, at = seq(-20, 20, by = 5), labels = seq(-20, 20, by = 5))
    axis(side = 4, at = seq(-5, 5, by = 5) * 4 - 35, labels = seq(-5, 5, by = 5))
    mtext("g", side = 4, line = 3, at = -30)

    par(oldpar)

## Comparison to Laplace approximation

An alternative way to marginalise over the continuous state process is the Laplace approximation, available in `RTMB` via the `random` argument of `MakeADFun()`. Instead of discretising the state space, we write the joint negative log-likelihood — the negative log of the joint density of observations and states — and declare the state sequence \(g_1, \dots, g_T\) as random effects. `RTMB` then integrates them out automatically using the Laplace approximation.

For the stochastic volatility model the joint negative log-likelihood is

\[ -\ell(\theta,\, g_{1:T}) = -\log f(g_1) - \sum_{t=2}^T \log f(g_t \mid g_{t-1}) - \sum_{t=1}^T \log f(y_t \mid g_t), \]

where \(g_1 \sim N(0,\, \sigma^2/(1-\phi^2))\), \(g_t \mid g_{t-1} \sim N(\phi g_{t-1},\, \sigma^2)\), and \(y_t \mid g_t \sim N(0,\, (\beta e^{g_t/2})^2)\).

    jnll = function(par) {
      getAll(par, dat)
      phi   = plogis(logit_phi); REPORT(phi)
      sigma = exp(log_sigma);    REPORT(sigma)
      beta  = exp(log_beta);     REPORT(beta)
      jnll = 0
      # stationary initial distribution: g_1 ~ N(0, sigma^2 / (1 - phi^2))
      jnll = jnll - dnorm(g[1], 0, sigma / sqrt(1 - phi^2), log = TRUE)
      # AR(1) state transitions: g_t | g_{t-1} ~ N(phi * g_{t-1}, sigma^2)
      jnll = jnll - sum(dnorm(g[-1], phi * g[-length(g)], sigma, log = TRUE))
      # observations: y_t | g_t ~ N(0, (beta * exp(g_t / 2))^2)
      jnll = jnll - sum(dnorm(y, 0, beta * exp(g / 2), log = TRUE))
      jnll
    }

The state sequence `g` is included in the parameter list and declared as `random` — `MakeADFun()` marginalises over it internally.

    dat = list(y = y)

    par_lap = list(
      logit_phi = qlogis(0.95),
      log_sigma  = log(0.3),
      log_beta   = log(1),
      g          = rep(0, n)   # random effects initialised at zero
    )

    obj_lap = MakeADFun(jnll, par_lap, random = "g", silent = TRUE)
    system.time(
      opt_lap <- nlminb(obj_lap$par, obj_lap$fn, obj_lap$gr)
    )
    #>    user  system elapsed
    #>   0.072   0.002   0.077
    mod_lap = report(obj_lap)

    mod_lap$phi    # true: 0.95
    #> [1] 0.9525517
    mod_lap$sigma  # true: 0.5
    #> [1] 0.4348222
    mod_lap$beta   # true: 2
    #> [1] 2.182106

The Laplace approach is typically faster because it avoids the expensive \(m \times m\) matrix operations of the discretisation: for a Markov state process the Hessian of the random-effect log-posterior is tridiagonal, so each Laplace step costs \(O(T)\) rather than \(O(m^2 T)\).

Comparing the two sets of estimates, \(\hat{\phi}\) and \(\hat{\sigma}\) differ slightly between the two approaches — only at the third decimal place here, and with a single dataset we cannot distinguish approximation error from ordinary sampling variation. In general, however, the Laplace approximation is not exact: it is only exact when the random-effect posterior is truly Gaussian given the fixed parameters, which is never strictly the case because \(\phi\) and \(\sigma\) enter the likelihood in a nonlinear way. Systematic discrepancies can grow substantially for non-Gaussian state processes such as Markov-switching autoregressive models, or for strongly non-Gaussian conditional distributions such as Bernoulli observations, where the posterior over \(g\) is multimodal or strongly skewed.

The discretisation approach makes no such assumption: increasing `m` reduces the quadrature error at the cost of computation time, giving full control over the approximation quality.

## References

Langrock, Roland. 2011. “On Some Special-Purpose Hidden Markov Models.” PhD thesis, University of Göttingen.