MCMCregressChange {MCMCpack} | R Documentation

## Markov Chain Monte Carlo for a linear Gaussian Multiple Changepoint Model

### Description

This function generates a sample from the posterior distribution of a linear Gaussian model with multiple changepoints. The function uses the Markov chain Monte Carlo method of Chib (1998). The user supplies data and priors, and a sample from the posterior distribution is returned as an mcmc object, which can be subsequently analyzed with functions provided in the coda package.

### Usage

    MCMCregressChange(
      formula,
      data = parent.frame(),
      m = 1,
      b0 = 0,
      B0 = 0,
      c0 = 0.001,
      d0 = 0.001,
      sigma.mu = NA,
      sigma.var = NA,
      a = NULL,
      b = NULL,
      mcmc = 1000,
      burnin = 1000,
      thin = 1,
      verbose = 0,
      seed = NA,
      beta.start = NA,
      P.start = NA,
      random.perturb = FALSE,
      WAIC = FALSE,
      marginal.likelihood = c("none", "Chib95"),
      ...
    )

### Arguments

`formula` | Model formula.
`data` | Data frame.
`m` | The number of changepoints.
`b0` | The prior mean of β. This can either be a scalar or a column vector with dimension equal to the number of betas. If this takes a scalar value, then that value will serve as the prior mean for all of the betas.
`B0` | The prior precision of β. This can either be a scalar or a square matrix with dimensions equal to the number of betas. If this takes a scalar value, then that value times an identity matrix serves as the prior precision of beta. Default value of 0 is equivalent to an improper uniform prior for beta.
`c0` | c_0/2 is the shape parameter for the inverse Gamma prior on σ^2 (the variance of the disturbances). The amount of information in the inverse Gamma prior is something like that from c_0 pseudo-observations.
`d0` | d_0/2 is the scale parameter for the inverse Gamma prior on σ^2 (the variance of the disturbances). In constructing the inverse Gamma prior, d_0 acts like the sum of squared errors from the c_0 pseudo-observations.
`sigma.mu` | The mean of the inverse Gamma prior on σ^2. sigma.mu and sigma.var allow users to choose the inverse Gamma prior by choosing its mean and variance.
`sigma.var` | The variacne of the inverse Gamma prior on σ^2. sigma.mu and sigma.var allow users to choose the inverse Gamma prior by choosing its mean and variance.
`a` | a is the shape1 beta prior for transition probabilities. By default, the expected duration is computed and corresponding a and b values are assigned. The expected duration is the sample period divided by the number of states.
`b` | b is the shape2 beta prior for transition probabilities. By default, the expected duration is computed and corresponding a and b values are assigned. The expected duration is the sample period divided by the number of states.
`mcmc` | The number of MCMC iterations after burnin.
`burnin` | The number of burn-in iterations for the sampler.
`thin` | The thinning interval used in the simulation. The number of MCMC iterations must be divisible by this value.
`verbose` | A switch which determines whether or not the progress of the sampler is printed to the screen. If `verbose` is greater than 0 the iteration number, the β vector, and the error variance are printed to the screen every `verbose`th iteration.
`seed` | The seed for the random number generator. If NA, the Mersenne Twister generator is used with default seed 12345; if an integer is passed it is used to seed the Mersenne twister. The user can also pass a list of length two to use the L'Ecuyer random number generator, which is suitable for parallel computation. The first element of the list is the L'Ecuyer seed, which is a vector of length six or NA (if NA a default seed of `rep(12345,6)` is used). The second element of list is a positive substream number. See the MCMCpack specification for more details.
`beta.start` | The starting values for the β vector. This can either be a scalar or a column vector with dimension equal to the number of betas. The default value of of NA will use the MLE estimate of β as the starting value. If this is a scalar, that value will serve as the starting value mean for all of the betas.
`P.start` | The starting values for the transition matrix. A user should provide a square matrix with dimension equal to the number of states. By default, draws from the `Beta(0.9, 0.1)` are used to construct a proper transition matrix for each raw except the last raw.
`random.perturb` | If TRUE, randomly sample hidden states whenever regularly sampled hidden states have at least one single observation state (SOS). SOS is a sign of overfitting in non-ergodic hidden Markov models.
`WAIC` | Compute the Widely Applicable Information Criterion (Watanabe 2010).
`marginal.likelihood` | How should the marginal likelihood be calculated? Options are: `none` in which case the marginal likelihood will not be calculated, and `Chib95` in which case the method of Chib (1995) is used.
`...` | further arguments to be passed

### Details

`MCMCregressChange` simulates from the posterior distribution of the linear regression model with multiple changepoints.

The model takes the following form:

y_t=x_t ' β_i + I(s_t=i)\varepsilon_{t},\;\; i=1, …, k

Where k is the number of states and I(s_t=i) is an indicator function that becomes 1 when a state at t is i and otherwise 0.

The errors are assumed to be Gaussian in each regime:

I(s_t=i)\varepsilon_{t} \sim \mathcal{N}(0, σ^2_i)

We assume standard, semi-conjugate priors:

β_i \sim \mathcal{N}(b_0,B_0^{-1}),\;\; i=1, …, k

And:

σ^{-2}_i \sim \mathcal{G}amma(c_0/2, d_0/2),\;\; i=1, …, k

Where β_i and σ^{-2}_i are assumed a priori independent.

The simulation proper is done in compiled C++ code to maximize efficiency.

### Value

An mcmc object that contains the posterior sample. This object can be summarized by functions provided by the coda package. The object contains an attribute `prob.state` storage matrix that contains the probability of state_i for each period, the log-likelihood of the model (`loglike`), and the log-marginal likelihood of the model (`logmarglike`).

### References

Jong Hee Park, 2012. “Unified Method for Dynamic and Cross-Sectional Heterogeneity: Introducing Hidden Markov Panel Models.” American Journal of Political Science.56: 1040-1054. <doi: 10.1111/j.1540-5907.2012.00590.x>

Sumio Watanabe. 2010. "Asymptotic equivalence of Bayes cross validation and widely applicable information criterion in singular learning theory" Journal of Machine Learning Research. 11: 3571-3594.

Siddhartha Chib. 1995. "Marginal Likelihood from the Gibbs Output." Journal of the American Statistical Association. 90: 1313-1321. <doi: 10.1016/S0304-4076(97)00115-2>

Siddhartha Chib. 1998. "Estimation and comparison of multiple change-point models." Journal of Econometrics. 86: 221-241. <doi: 10.1080/01621459.1995.10476635>

### See Also

`plotState`, `plotChangepoint`

### Examples

    ## Not run:
    set.seed(1119)
    n <- 100
    x1 <- runif(n)
    true.beta1 <- c(2, -2)
    true.beta2 <- c(0,  2)
    true.Sigma <- c(1, 2)
    true.s <- rep(1:2, each=n/2)

    mu1 <- cbind(1, x1[true.s==1])%*%true.beta1
    mu2 <- cbind(1, x1[true.s==2])%*%true.beta2

    y <- as.ts(c(rnorm(n/2, mu1, sd=sqrt(true.Sigma[1])), rnorm(n/2, mu2, sd=sqrt(true.Sigma[2]))))
    formula=y ~ x1

    ols1 <- lm(y[true.s==1] ~x1[true.s==1])
    ols2 <- lm(y[true.s==2] ~x1[true.s==2])

    ## prior
    b0 <- 0
    B0 <- 0.1
    sigma.mu=sd(y)
    sigma.var=var(y)

    ## models
    model0 <-  MCMCregressChange(formula, m=0, b0=b0, B0=B0, mcmc=100, burnin=100,
               sigma.mu=sigma.mu, sigma.var=sigma.var, marginal.likelihood="Chib95")
    model1 <-  MCMCregressChange(formula, m=1, b0=b0, B0=B0, mcmc=100, burnin=100,
               sigma.mu=sigma.mu, sigma.var=sigma.var, marginal.likelihood="Chib95")
    model2 <-  MCMCregressChange(formula, m=2, b0=b0, B0=B0, mcmc=100, burnin=100,
               sigma.mu=sigma.mu, sigma.var=sigma.var, marginal.likelihood="Chib95")
    model3 <-  MCMCregressChange(formula, m=3, b0=b0, B0=B0, mcmc=100, burnin=100,
               sigma.mu=sigma.mu, sigma.var=sigma.var, marginal.likelihood="Chib95")
    model4 <-  MCMCregressChange(formula, m=4, b0=b0, B0=B0, mcmc=100, burnin=100,
               sigma.mu=sigma.mu, sigma.var=sigma.var, marginal.likelihood="Chib95")
    model5 <-  MCMCregressChange(formula, m=5, b0=b0, B0=B0, mcmc=100, burnin=100,
               sigma.mu=sigma.mu, sigma.var=sigma.var, marginal.likelihood="Chib95")

    print(BayesFactor(model0, model1, model2, model3, model4, model5))
    plotState(model1)
    plotChangepoint(model1)

    ## End(Not run)

[Package MCMCpack version 1.5-0 Index]