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Re: Hidden Markov Model and Underflow problem

To: "'Yingfu Xie'" <Yingfu.Xie@sekon.slu.se>, "'s-news@lists.biostat.wustl.edu'" <s-news@lists.biostat.wustl.edu>
Subject: Re: Hidden Markov Model and Underflow problem
From: "Liaw, Andy" <andy_liaw@merck.com>
Date: Mon, 12 Jul 2004 12:50:08 -0400
I believe the `msm' package for R on CRAN has facility for HMM.  I seem to
recall that one of Jim Lindsay's packages (on his web page:
http://popgen0146uns50.unimaas.nl/~jlindsey/rcode.html) also has HMM
facilities, but I'm not certain.

Andy

> From: Yingfu Xie
> 
> Hello, all,
> 
> Does anybody know any function or package in S-plus or R for 
> estimation of a
> Hidden Markov Model(HMM)? 
> 
> I have tried to write one by myself. First I compute the 
> likelihood function
> recursively, then use nlminb or optim to maximize it. 
> However, my algorithm
> suffers a serious underflow problem, i.e., the value of the 
> likelihood of
> the model at some stage of the algorithm is too small to be 
> distinguishable
> from zero. Simply multiplying by a large constant or taking algorithm
> doesn't work. The scaling method introduced in the monograph 
> by MacDonald
> and Zucchini (Hidden Markov and Other Models for Discrete-Valued Time
> Series) isn't satisfactory either. Any idea how can I improve it?  
> Thanks a lot in advance!
> 
> Regards,
> Yingfu
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