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20 - Algorithms for HMMs

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Questions and Answers

What are the two key observations of the naive approach for the evaluation problem?

Only depends on the current state and only depends on the current state (Markov chain).

What is the main issue that arises when trying to compute the answer in the evaluation problem?

Combinatorial explosion due to the large number of state sequences to sum over.

How does the Forward Algorithm approach the evaluation problem?

By using Dynamic Programming and merging all states with the current state.

What does the Forward-Backward Algorithm aim to compute?

It aims to compute the most likely sequence of labels using both forward and backward variables. Signup and view all the answers

In the context of the decoding problem, how is the Viterbi Algorithm related to the evaluation problem?

The decoding problem is very similar to the evaluation problem. Signup and view all the answers

What is the main drawback mentioned if one estimates all sequences in the decoding problem using Bayes' rule?

The resulting sequence may contain invalid transitions with probability 0. Signup and view all the answers

What is the purpose of the Baum-Welch Algorithm?

Optimize the parameters using an EM procedure Signup and view all the answers

In the Baum-Welch Algorithm, what is done after initializing the guesses for the parameters?

Estimate the hidden variables using the Forward-Backward estimates Signup and view all the answers

What is the runtime complexity of the Viterbi algorithm?

(number of states) * (number of steps) Signup and view all the answers

What is the main idea behind Beam Search as an alternative to Viterbi?

To keep only the most likely states in each step Signup and view all the answers

What is an alternative to Beam Search for optimizing results on the Viterbi graph?

A*-search Signup and view all the answers

Why is Beam Search used in the context of Viterbi?

To handle the challenge of many states having low probabilities Signup and view all the answers