MCMCnegbinChange {MCMCpack} | R Documentation

## Markov Chain Monte Carlo for Negative Binomial Regression Changepoint Model

### Description

This function generates a sample from the posterior distribution of a Negative Binomial regression model with multiple changepoints. For the changepoints, the sampler 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

    MCMCnegbinChange(
      formula,
      data = parent.frame(),
      m = 1,
      fixed.m = TRUE,
      b0 = 0,
      B0 = 1,
      a = NULL,
      b = NULL,
      e = 2,
      f = 2,
      g = 10,
      burnin = 1000,
      mcmc = 1000,
      thin = 1,
      verbose = 0,
      seed = NA,
      beta.start = NA,
      P.start = NA,
      rho.start = NA,
      rho.step,
      nu.start = NA,
      marginal.likelihood = c("none", "Chib95"),
      ...
    )

### Arguments

`formula` | Model formula.
`data` | Data frame.
`m` | The number of changepoints.
`fixed.m` | A logical indicator for whether or not the number of changepoints in the sampler should be exactly equal to `m` or if that is simply an upper bound. Setting `fixed.m` to `FALSE` is equivalent to assuming a weak-limit approximation to a Dirichlet process mixture.
`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.
`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.
`e` | The hyperprior for the distribution ρ See details.
`f` | The hyperprior for the distribution ρ. See details.
`g` | The hyperprior for the distribution ρ. See details.
`burnin` | The number of burn-in iterations for the sampler.
`mcmc` | The number of Metropolis 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 current beta vector, and the Metropolis acceptance rate 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 value for the β vector. 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 starting value for all of the betas. The default value of NA will use the maximum likelihood estimate of β as the starting value for all regimes.
`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.
`rho.start` | The starting value for the ρ variable. This can either be a scalar or a column vector with dimension equal to the number of regimes. If the value is scalar, it will be used for all regimes. The default value is a vector of ones.
`rho.step` | Tuning parameter for the slice sampling approach to sampling rho. Determines the size of the step-out used to find the correct slice to draw from. Lower values are more accurate, but will take longer (up to a fixed searching limit). Default is 0.1.
`nu.start` | The starting values for the random effect, ν. The default value is a vector of ones.
`marginal.likelihood` | How should the marginal likelihood be calculated? Options are: `none` in which case the marginal likelihood will not be calculated or `Laplace` in which case the Laplace approximation (see Kass and Raftery, 1995) is used.
`...` | further arguments to be passed.

### Details

`MCMCnegbinChange`simulates from the posterior distribution of a Negative Binomial regression model with multiple changepoints using the methods of Chib (1998) and Fruehwirth-Schnatter et al (2009). The details of the model are discussed in Blackwell (2017).

The model takes the following form:

y_t \sim \mathcal{P}oisson(ν_tμ_t)

μ_t = x_t ' β_m,\;\; m = 1, …, M

ν_t \sim \mathcal{G}amma(ρ_m, ρ_m)

Where M is the number of states and β_m and ρ_m are parameters when a state is m at t.

We assume Gaussian distribution for prior of β:

β_m \sim \mathcal{N}(b_0,B_0^{-1}),\;\; m = 1, …, M

And:

p_{mm} \sim \mathcal{B}eta(a, b),\;\; m = 1, …, M

Where M is the number of states.

The overdispersion parameters have a prior with the following form:

f(ρ_m|e,f,g) \propto ρ^{e-1}(ρ + g)^{-(e+f)}

The model is simulated via blocked Gibbs conditonal on the states. The β being simulated via the auxiliary mixture sampling method of Fuerhwirth-Schanetter et al. (2009). The ρ is updated via slice sampling. The ν_i are updated their (conjugate) full conditional, which is also Gamma. The states are updated as in Chib (1998)

### Value

An mcmc object that contains the posterior sample. This object can be summarized by functions provided by the coda package.

### References

Andrew D. Martin, Kevin M. Quinn, and Jong Hee Park. 2011. “MCMCpack: Markov Chain Monte Carlo in R.”, Journal of Statistical Software. 42(9): 1-21. https://www.jstatsoft.org/v42/i09/.

Daniel Pemstein, Kevin M. Quinn, and Andrew D. Martin. 2007. Scythe Statistical Library 1.0. http://scythe.lsa.umich.edu.

Sylvia Fruehwirth-Schnatter, Rudolf Fruehwirth, Leonhard Held, and Havard Rue. 2009. “Improved auxiliary mixture sampling for hierarchical models of non-Gaussian data”, Statistics and Computing 19(4): 479-492. <doi:10.1007/s11222-008-9109-4>

Matthew Blackwell. 2017. “Game Changers: Detecting Shifts in Overdispersed Count Data,” Political Analysis 26(2), 230-239. <doi:10.1017/pan.2017.42>

### See Also

`MCMCpoissonChange`, `plotState`, `plotChangepoint`

### Examples

     ## Not run:
       n <- 150
       reg <- 3
       true.s <- gl(reg, n/reg, n)
       rho.true <- c(1.5, 0.5, 3)
       b0.true <- c(1, 3, 1)
       b1.true <- c(1, -2, 2)
       x1 <- runif(n, 0, 2)
       nu.true <- rgamma(n, rho.true[true.s], rho.true[true.s])
       mu <- nu.true * exp(b0.true[true.s] + x1 * b1.true[true.s])
       y <- rpois(n, mu)

       posterior <- MCMCnegbinChange(y ~ x1, m = 2, verbose = 1000,
                              marginal.likelihood = "Chib95",
                              e = 2, f = 2, g = 10,
                              b0 = rep(0, 2), B0 = (1/9) * diag(2),
                              rho.step = rep(0.75, times = 3),
                              seed = list(NA, 2))

       par(mfrow=c(attr(posterior, "m") + 1, 1), mai=c(0.4, 0.6, 0.3, 0.05))
       plotState(posterior, legend.control = c(1, 0.6))
       plotChangepoint(posterior, verbose = TRUE, ylab="Density",
      start=1, overlay=TRUE)

       open.ended <- MCMCnegbinChange(y ~ x1, m = 10, verbose = 1000,
                              fixed.m = FALSE, mcmc = 2000, burnin = 2000,
                              e = 2, f = 2, g = 10,
                              a = 100, b = 1,
                              b0 = rep(0, 2), B0 = (1/9) * diag(2),
                              rho.step = 0.75,
                              seed = list(NA, 2))

       plotState(open.ended, legend.control = c(1, 0.6))

    ## End(Not run)

[Package MCMCpack version 1.5-0 Index]