(data structure)

**Definition:**
A variant of a *finite state machine* having a set of *states*, Q, an output *alphabet*, O, transition probabilities, A, output probabilities, B, and initial state probabilities, Π. The current state is not observable. Instead, each state produces an output with a certain probability (B). Usually the states, Q, and outputs, O, are understood, so an HMM is said to be a triple, (A, B, Π).

**Formal Definition:** After Michael Cohen's lectures for CN760.

- A = {a
_{ij}= P(q_{j}at t+1 | q_{i}at t)}, where P(a | b) is the conditional probability of a given b, t ≥ 1 is time, and q_{i}∈ Q.

Informally, A is the probability that the next state is q_{j}given that the current state is q_{i}. - B = {b
_{ik}= P(o_{k}| q_{i})}, where o_{k}∈ O.

Informally, B is the probability that the output is o_{k}given that the current state is q_{i}. - Π = {p
_{i}= P(q_{i}at t=1)}.

**Also known as** HMM.

**Generalization** (I am a kind of ...)

*finite state machine*.

**Aggregate parent** (I am a part of or used in ...)

*Baum Welch algorithm*, *Viterbi algorithm*.

**See also**
*Markov chain*.

*Note:
Computing a model given sets of sequences of observed outputs is very difficult, since the states are not directly observable and transitions are probabilistic. One method is the Baum Welch algorithm. *

Although the states cannot, by definition, be directly observed, the most likely sequence of sets for a given sequence of observed outputs can be computed in O(nt), where n is the number of states and t is the length of the sequence. One method is the *Viterbi algorithm*.

Thanks to Arvind <uk_arvind@mail.utexas.edu> May 2002.

* Named after Andrei Andreyevich Markov (1856 - 1922), who studied poetry and other texts as stochastic sequences of characters.*

Author: PEB

**L. E. Baum**, *An inequality and associated maximization technique in statistical estimation for probabilistic functions of Markov processes*, Inequalities, 3:1-8, 1972.

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If you have suggestions, corrections, or comments, please get in touch with Paul Black.

Entry modified 14 August 2008.

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Cite this as:

Paul E. Black, "hidden Markov model", in
*Dictionary of Algorithms and Data Structures* [online], Paul E. Black, ed. 14 August 2008. (accessed TODAY)
Available from: https://www.nist.gov/dads/HTML/hiddenMarkovModel.html