Baum_Welch_algorithm: Estimation Using the Baum-Welch Algorithm
In HMMpa: Analysing Accelerometer Data Using Hidden Markov Models

Baum_Welch_algorithm

R Documentation

Estimation Using the Baum-Welch Algorithm

Description

Estimates the parameters of a (non-stationary) discrete-time hidden Markov model. The Baum-Welch algorithm is a version of the EM (Estimation/Maximization) algorithm. See MacDonald & Zucchini (2009, Paragraph 4.2) for further details.

Usage

Baum_Welch_algorithm(
  x,
  m,
  delta,
  gamma,
  distribution_class,
  distribution_theta,
  discr_logL = FALSE,
  discr_logL_eps = 0.5,
  BW_max_iter = 50,
  BW_limit_accuracy = 0.001,
  BW_print = TRUE,
  Mstep_numerical = FALSE,
  DNM_limit_accuracy = 0.001,
  DNM_max_iter = 50,
  DNM_print = 2
)

Arguments

`x`	a vector object containing the time-series of observations that are assumed to be realizations of the (hidden Markov state dependent) observation process of the model.
`m`	integer; a (finite) number of states in the hidden Markov chain.
`delta`	vector object containing starting values for the marginal probability distribution of the `m` states of the Markov chain at the time point `t=1` for the Baum-Welch algorithm.
`gamma`	a matrix (`ncol=nrow=m`) containing starting values for the transition matrix of the hidden Markov chain
`distribution_class`	a single character string object with the abbreviated name of the `m` observation distributions of the Markov dependent observation process. The following distributions are supported: Poisson (`pois`); generalized Poisson (`genpois`, parameter estimation via the Baum-Welch algorithm is only supported if the M-step is performed numerically, i.e. if `Mstep_numerical = TRUE`); normal (`norm`)
`distribution_theta`	a list object containing starting values for the parameters of the `m` observation distributions of the observation process that are dependent on the hidden Markov state.
`discr_logL`	a logical object indicating whether the discrete log-likelihood should be used (for `distribution_class="norm"`) for estimating the model specific parameters instead of the general log-likelihood. See MacDonald & Zucchini (2009, Paragraph 1.2.3) for further details. Default value is `FALSE`.
`discr_logL_eps`	a single numerical value to approximately determine the discrete likelihood for a hidden Markov model based on nomal distributions (for `"norm"`). Default value is `0.5`. See MacDonald & Zucchini (2009, Paragraph 1.2.3) for further details.
`BW_max_iter`	a single numerical value representing the maximum number of iterations in the Baum-Welch algorithm. Default value is `50`.
`BW_limit_accuracy`	a single numerical value representing the convergence criterion of the Baum-Welch algorithm. Default value is `0.001`.
`BW_print`	a logical object indicating whether the log-likelihood at each iteration-step shall be printed. Default value is `TRUE`.
`Mstep_numerical`	a logical object indicating whether the Maximization Step of the Baum-Welch algorithm shall be performed by numerical maximization using the nlm-function. Default value is `FALSE`.
`DNM_limit_accuracy`	a single numerical value representing the convergence criterion of the numerical maximization algorithm using the nlm-function (used to perform the M-step of the Baum-Welch-algorithm). Default value is `0.001`.
`DNM_max_iter`	a single numerical value representing the maximum number of iterations of the numerical maximization using the nlm-function (used to perform the M-step of the Baum-Welch-algorithm). Default value is `50`.
`DNM_print`	a single numerical value to determine the level of printing of the `nlm`-function. See `nlm`-function for further informations. The value `0` suppresses, that no printing will be outputted. Default value is `2` for full printing.

Value

Baum_Welch_algorithm returns a list containing the estimated parameters of the hidden Markov model and other components. See MacDonald & Zucchini (2009, Paragraph 4.2) for further details on the calculated objects within this algorithm.

x: input time-series of observations.
m: input number of hidden states in the Markov chain.
zeta: a (T,m)-matrix (when T indicates the length/size of the observation time-series and m the number of states of the HMM) containing probabilities (estimates of the conditional expectations of the missing data given the observations and the estimated model specific parameters) calculated by the algorithm. See MacDonald & Zucchini (2009, Paragraph 4.2.2) for further details.
eta: a (T,m,m)-dimensional-array (when T indicates the length of the observation time-series and m the number of states of the HMM) containing probabilities (estimates of the conditional expectations of the missing data given the observations and the estimated model specific parameters) calculated by the algorithm. See MacDonald & Zucchini (2009, Paragraph 4.2.2) for further details.
logL: a numerical value representing the logarithmized likelihood calculated by the forward_backward_algorithm.
iter: number of performed iterations.
BIC: a numerical value representing the Bayesian information criterion for the hidden Markov model with estimated parameters.
delta: a vector object containing the estimates for the marginal probability distribution of the m states of the Markov chain at time-point point t=1.
gamma: a matrix containing the estimates for the transition matrix of the hidden Markov chain.
...: other input values (as arguments above). In the case that the algorithm stops before the targeted accuracy or the maximum number of iterations has been reached, further values are displayed and the estimates from the last successful iteration step are saved.

Author(s)

The basic algorithm for a Poisson-HMM is provided by MacDonald & Zucchini (2009, Paragraph 4.2, Paragraph A.2.3). Extension and implementation by Vitali Witowski (2013).

References

Baum, L., Petrie, T., Soules, G., Weiss, N. (1970). A maximization technique occurring in the statistical analysis of probabilistic functions of markov chains. The annals of mathematical statistics, vol. 41(1), 164–171.

Dempster, A., Laird, N., Rubin, D. (1977). Maximum likelihood from incomplete data via the EM algorithm. Journal of the Royal Statistical Society. Series B (Methodological), vol. 39(1), 1–38.

MacDonald, I. L., Zucchini, W. (2009) Hidden Markov Models for Time Series: An Introduction Using R, Boca Raton: Chapman & Hall.

Examples

x <- c(1,16,19,34,22,6,3,5,6,3,4,1,4,3,5,7,9,8,11,11,
  14,16,13,11,11,10,12,19,23,25,24,23,20,21,22,22,18,7,
  5,3,4,3,2,3,4,5,4,2,1,3,4,5,4,5,3,5,6,4,3,6,4,8,9,12,
  9,14,17,15,25,23,25,35,29,36,34,36,29,41,42,39,40,43,
  37,36,20,20,21,22,23,26,27,28,25,28,24,21,25,21,20,21,
  11,18,19,20,21,13,19,18,20,7,18,8,15,17,16,13,10,4,9,
  7,8,10,9,11,9,11,10,12,12,5,13,4,6,6,13,8,9,10,13,13,
  11,10,5,3,3,4,9,6,8,3,5,3,2,2,1,3,5,11,2,3,5,6,9,8,5,
  2,5,3,4,6,4,8,15,12,16,20,18,23,18,19,24,23,24,21,26,
  36,38,37,39,45,42,41,37,38,38,35,37,35,31,32,30,20,39,
  40,33,32,35,34,36,34,32,33,27,28,25,22,17,18,16,10,9,
  5,12,7,8,8,9,19,21,24,20,23,19,17,18,17,22,11,12,3,9,
  10,4,5,13,3,5,6,3,5,4,2,5,1,2,4,4,3,2,1) 
  
  # Assumptions (number of states, probability vector, 
  # transition matrix, and distribution parameters)
  
  m <- 4
  delta <- c(0.25,0.25,0.25,0.25)
  gamma <- 0.7 * diag(m) + rep(0.3 / m)
  distribution_class <- "pois"
  distribution_theta <- list(lambda = c(4,9,17,25))
  
  # Estimation of a HMM using the Baum-Welch algorithm
  
    trained_HMM_with_m_hidden_states <- 
     Baum_Welch_algorithm(x = x, 
                          m = m, 
                      delta = delta, 
                      gamma = gamma,
         distribution_class = distribution_class, 
         distribution_theta = distribution_theta)
         
 print(trained_HMM_with_m_hidden_states)

HMMpa documentation built on April 3, 2025, 10:29 p.m.