Hidden Markov Models (HMM) are stochastic methods to model temporal and sequence
data. They are especially known for their application in
temporal pattern recognition such
as speech,
handwriting,
gesture recognition,
partofspeech tagging,
musical score following,
partial discharges
and bioinformatics.
This code has also been incorporated in
Accord.NET Framework, which includes
the latest version of this code plus many other
statistics and machine learning
tools.
Contents

Introduction

Definition

Notation

Canonical problems

Choosing the structure

Algorithms

Evaluation

Decoding

Learning

Using the code

Remarks

Known issues

Acknowledgements

See also

References
Hidden Markov Models were first described in a series of statistical papers by
Leonard E. Baum and other authors in the second half of the 1960s. One of the
first applications of HMMs was
speech recognition,
starting in the mid1970s. Indeed, one of the most comprehensive explanations on
the topic was published in “A
Tutorial On Hidden Markov Models And Selected Applications in Speech Recognition”,
by Lawrence R. Rabiner in 1989. In the second half of the 1980s, HMMs began to be
applied to the analysis of biological sequences, in particular
DNA. Since then, they have become
ubiquitous in the field of
bioinformatics.
Dynamical systems of discrete nature assumed to be governed by a Markov chain
emits a sequence of observable outputs. Under the Markov assumption,
it is also assumed
that the latest output depends only on the current state of the system. Such states
are often not known from the observer when only the output values are observable.
Hidden Markov Models attempt to model such systems and allow, among other things,
(1) to infer the most likely sequence of states that produced a given output sequence,
to (2) infer which will be the most likely next state (and thus predicting the next
output) and (3) calculate the probability that a given sequence of outputs originated
from the system (allowing the use of hidden Markov models for sequence classification).
The “hidden” in Hidden Markov Models comes from the fact that the observer does
not know in which state the system may be in, but has only a probabilistic insight
on where it should be.
Hidden Markov Models can be seem as finite state machines where for each sequence
unit observation there is a state transition and, for each state, there is a output
symbol emission.
Traditionally, HMMs have been defined by the following quintuple:
where
 N is the number of states for the model
 M is the number of distinct observations symbols per state, i.e. the discrete
alphabet size.
 A is the NxN state transition probability distribution given in the form
of a matrix A = {a_{ij}}
 B is the NxM observation symbol probability distribution given in the form
of a matrix B = {b_{j}(k)}
 π is the initial state distribution vector π = {π_{i}}
Note that, if we opt out the structure parameters M and N we have the more often
used compact notation
There are three canonical
problems associated with hidden Markov models, which I'll quote from Wikipedia:
 Given the parameters of the model, compute the probability of a particular
output sequence. This requires summation over all possible state sequences,
but can be done efficiently using the
Forward algorithm,
which is a form of
dynamic programming.
 Given the parameters of the model and a particular output sequence, find
the state sequence that is most likely to have generated that output sequence.
This requires finding a maximum over all possible state sequences, but can similarly
be solved efficiently by the
Viterbi algorithm.
 Given an output sequence or a set of such sequences, find the most likely
set of state transition and output probabilities. In other words, derive the
maximum likelihood
estimate of the parameters of the HMM given a dataset of output sequences. No
tractable algorithm is known for solving this problem exactly, but a local maximum
likelihood can be derived efficiently using the
BaumWelch algorithm
or the
BaldiChauvin algorithm. The
BaumWelch algorithm
is an example of a
forwardbackward
algorithm, and is a special case of the
Expectationmaximization
algorithm.
The solution for those problems are exactly what makes Hidden Markov Models useful.
The ability to learn from the data (using the solution of problem 3) and then become
able to make predictions (solution to problem 2) and able to classify sequences
(solution of problem 2) is nothing but
applied machine learning.
From this perspective, HMMs can just be seem as
supervisioned sequence
classifiers and sequence predictors with some other useful interesting properties.
Choosing the structure for a hidden Markov model is not always obvious. The number
of states depend on the application and to what interpretation one is willing to
give to the hidden states. Some domain knowledge is required to build a suitable
model and also to choose the initial parameters that an HMM can take. There is also
some trial and error involved, and there are sometimes complex tradeoffs that have
to be made between model complexity and difficulty of learning, just as is the case
with most machine learning techniques.
Additional information can be found on
http://www.cse.unsw.edu.au/~waleed/phd/tr9806/node12.html.
The solution to the three canonical problems are the algorithms that makes HMMs
useful. Each of the three problems are described in the three subsections below.
The first canonical problem is the evaluation of the probability of a particular
output sequence. It can be efficiently computed using either the Viterbiforward
or the Forward algorithms, both of which are forms of dynamic programming.
The Viterbi algorithm originally computes the most likely sequence of states
which has originated a sequence of observations. In doing so, it is also able to
return the probability of traversing this particular sequence of states. So to obtain
Viterbi probabilities, please refer to the Decoding problem referred below.
The Forward algorithm, unlike the Viterbi algorithm, does not find a particular
sequence of states; instead it computes the probability that any sequence
of states has produced the sequence of observations. In both algorithms, a matrix
is used to store computations about the possible state sequence paths that the model
can assume. The forward algorithm also plays a key role in the Learning problem,
and is thus implemented as a separate method.
</span>
The second canonical problem is the discovery of the most likely sequence of
states that generated a given output sequence. This can be computed efficiently
using the Viterbi algorithm. A trackback is used to detect the maximum probability
path travelled by the algorithm. The probability of travelling such sequence is
also computed in the process.
</span>
The third and last problem is the problem of learning the most likely parameters
that best models a system given a set of sequences originated from this system.
Most implementations I've seem did not consider the problem of learning from a set
of sequences, but only from a single sequence at a time. The algorithm below, however,
is fully suitable to learn from a set of sequences and also uses scaling,
which is another thing I have not seem in other implementations.
The source code follows the original algorithm by Rabiner (1989). There are,
however, some known issues with the algorithms detailed in Rabiner's paper. More
information about those issues is available in a next section of this article entitled
“Remarks”.
</span>
Lets suppose we have gathered some sequences from a system we wish to model.
The sequences are expressed as a integer array such as:
int[][] sequences = new int[][]
{
new int[] { 0,1,1,1,1,1,1 },
new int[] { 0,1,1,1 },
new int[] { 0,1,1,1,1 },
new int[] { 0,1, },
new int[] { 0,1,1 },
};
For us, it can be obvious to see that the system is outputting sequences that always
start with a zero and have one or more ones at the end. But lets try to fit a Hidden
Markov Model to predict those sequences.
HiddenMarkovModel hmm = new HiddenMarkovModel(2, 2);
hmm.Learn(sequences, 0.01);
Once the model is trained, lets test to see if it recognizes some sequences:
double l1 = hmm.Evaluate(new int[] { 0, 1 }); double l2 = hmm.Evaluate(new int[] { 0, 1, 1, 1 });
double l3 = hmm.Evaluate(new int[] { 1, 1 }); double l4 = hmm.Evaluate(new int[] { 1, 0, 0, 0 });
Of course the model performs well as this a rather simple example. A more useful
test case would consist of allowing for some errors in the input sequences in the
hope that the model will become more tolerant to measurement errors.
int[][] sequences = new int[][]
{
new int[] { 0,1,1,1,1,0,1,1,1,1 },
new int[] { 0,1,1,1,0,1,1,1,1,1 },
new int[] { 0,1,1,1,1,1,1,1,1,1 },
new int[] { 0,1,1,1,1,1 },
new int[] { 0,1,1,1,1,1,1 },
new int[] { 0,1,1,1,1,1,1,1,1,1 },
new int[] { 0,1,1,1,1,1,1,1,1,1 },
};
HiddenMarkovModel hmm = new HiddenMarkovModel(2, 3);
hmm.Learn(sequences, 0.0001);
double l1 = hmm.Evaluate(new int[] { 0,1 }); double l2 = hmm.Evaluate(new int[] { 0,1,1,1 });
double l3 = hmm.Evaluate(new int[] { 1,1 }); double l4 = hmm.Evaluate(new int[] { 1,0,0,0 });
double l5 = hmm.Evaluate(new int[] { 0,1,0,1,1,1,1,1,1 }); double l6 = hmm.Evaluate(new int[] { 0,1,1,1,1,1,1,0,1 });
We can see that, despite having a very low probability, the likelihood values for
the sequences containing a simulated measurement error are greater than the likelihoods
for the sequences which do not follow the sequence structure at all.
In a
subsequent article, we will see that those low values for the likelihoods will
not be a problem because
HMMs are often used in sets to form sequence classifiers. When used in such
configurations, what really matters is which HMM returns the highest probability
among others in the set.
A practical issue in the use of Hidden Markov Models to model long sequences
is the numerical scaling of conditional probabilities. The probability of observing
a long sequence given most models is extremely small, and the use of these extremely
small numbers in computations often leads to numerical instability, making application
of HMMs to genome length sequences quite challenging.
There are two common approaches to dealing with small conditional probabilities.
One approach is to rescale the conditional probabilities using carefully designed
scaling factors, and the other approach is to work with the logarithms of the conditional
probabilities. For more information on using logarithms please see the work entitled
“Numerically
Stable Hidden Markov Model Implementation”, by Tobias P. Mann.
The code on this article is based on the Tutorial by Rabiner.
There are, however, some problems with the scaling and other algorithms. An errata
depicting all issues is available in the website “An
Erratum for ‘A Tutorial on Hidden Markov Models and Selected Applications in Speech
Recognition’” and is maintained by Ali Rahimi. I have not yet verified
if the implementation presented here also suffers from the same mistakes explained
there. This code has worked well under many situations, but I cannot guarantee its
perfectness. Please use at your own risk.
Thanks to Guilherme C. Pedroso, for the help with the BaumWelch generalization
for multiple input sequences. He has also cowritten a very interesting article
using hidden Markov models for gesture recognition, entitled “Automatic
Recognition of Finger Spelling for LIBRAS based on a TwoLayer Architecture”
published in the 25th Symposium On Applied Computing (ACM SAC 2010).

L. R. Rabiner, "A
Tutorial on Hidden Markov Models, and Selected Applications in Speech Recognition,"
Proc. IEEE, Vol. 77, No. 2, pp. 257286, Feb. 1989.

Warakagoda, Narada D., “A
Hybrid ANNHMM ASR system with NN based adaptive preprocessing”, Norges
Tekniske Høgskole, 2006.

A. Rahimim, “An
erratum for: A tutorial on hidden Markov models and selected applications in
speech recognition”, World Wide Web. Available in:
http://xenia.media.mit.edu/rahimi/rabiner/rabinererrata/rabinererrata.html.

T. P. Mann, "Numerically
stable hidden markov model implementation," World Wide Web. Available
in: http://www.phrap.org/compbio/mbt599/hmm_scaling_revised.pdf

Wikipedia contributors, "Hidden
Markov model,"
Wikipedia, The Free
Encyclopedia, http://en.wikipedia.org/wiki/Hidden_Markov_model (accessed
March, 2010).