Control parameters for the EM algorithm
Set control parameters for the EM algorithm.
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The maximum number of iterations.
The tolerance level for convergence. See Details.
Sets the convergence criterion to "relative" or "absolute" change of the log-likelihood. See Details.
This is used for a (limited) random initialization of the parameters. See Details.
Type of classification to states used. See Details.
crit sets the convergence criterion to either the
relative change in the log-likelihood or the absolute change in the
log-likelihood. The relative likelihood criterion (the default) assumes
convergence on iteration i when
(log L(i) - log L(i-1))/(log L(i-1)) < tol.
The absolute likelihood criterion assumes convergence on iteration
i when (log L(i) - log L(i-1)) < tol.
crit="absolute" to invoke the latter
convergence criterion. Note that in that case, optimal values of the
tol scale with the value of the log-likelihood
(and these are not changed automagically).
random.start This is used for a (limited) random
initialization of the parameters. In particular, if
random.start=TRUE, the (posterior) state probabilities are
randomized at iteration 0 (using a uniform distribution), i.e. the
γ variables (Rabiner, 1989) are sampled from the Dirichlet
distribution with a (currently fixed) value of
α=0.1; this results in values for each row of γ
that are quite close to zero and one; note that when these values are
chosen at zero and one, the initialization is similar to that used in
kmeans. Random initialization is useful when no initial parameters can be
given to distinguish between the states. It is also useful for repeated
estimation from different starting values.
classification is used to choose either soft (default) or
hard classification of observations to states. When using soft classification, observations
are assigned to states with a weight equal to the posterior probability of
the state. When using hard classification, observations are assigned to states
according to the maximum a posteriori (MAP) states (i.e., each observation
is assigned to one state, which is determined by the Viterbi algorithm in the
depmix models). As a result, the EM algorithm will find a local
maximum of the classification likelihood (Celeux & Govaert, 1992).
Warning: hard classification is an experimental feature,
especially for hidden Markov models, and its use is currently not advised.
em.control returns a list of the control parameters.
Ingmar Visser & Maarten Speekenbrink
Lawrence R. Rabiner (1989). A tutorial on hidden Markov models and selected applications in speech recognition. Proceedings of IEEE, 77-2, p. 267-295.
Gilles Celeux and Gerard Govaert (1992). A classification EM algorithm for clustering and two stochastic versions. Computational Statistics and Data Analysis, 14, p. 315-332.
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# using "hard" assignment of observations to the states, we can maximise the # classification likelihood instead of the usual marginal likelihood data(speed) mod <- depmix(list(rt~1,corr~1),data=speed,nstates=2, family=list(gaussian(),multinomial("identity")),ntimes=c(168,134,137)) set.seed(1) # fit the model by calling fit fmod <- fit(mod,emcontrol=em.control(classification="hard")) # can get rather different solutions with different starting values... set.seed(3) fmod2 <- fit(mod,emcontrol=em.control(classification="hard"))
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