Computation Complexity and Forward Algorithm in Hidden Markov Models

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Questions and Answers

What is the goal of the likelihood computation in hidden Markov models?

  • To find the optimal state sequence that maximizes the likelihood of the observation sequence
  • To estimate the parameters of the hidden Markov model
  • To determine the probability of the observation sequence given the hidden Markov model (correct)
  • To determine the most likely state sequence given the observation sequence

What is the formula used to compute the likelihood of the observation sequence $X = (x_1, x_2, ..., x_T)$ given the hidden Markov model $M$?

  • $p(X|M) = \prod_{t=1}^T P(x_t|q_t)$
  • $p(X|M) = P(q_1)P(x_1|q_1) \prod_{t=2}^T P(q_t|q_{t-1})P(x_t|q_t)$
  • $p(X|M) = \sum_{Q\in Q} P(X, Q|M)$ (correct)
  • $p(X|M) = \max_{Q\in Q} P(X, Q|M)

How many possible state sequences $Q = (q_1, q_2, ..., q_T)$ are there for an observation sequence of length $T$ in a hidden Markov model with $N$ states?

  • $N$
  • $N^2$
  • $T^N$
  • $N^T$ (correct)

Which of the following is a key challenge in computing the likelihood of the observation sequence in a hidden Markov model?

<p>The need to sum over all possible state sequences (C)</p>
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What is the purpose of the forward algorithm in hidden Markov models?

<p>To compute the likelihood of the observation sequence given the hidden Markov model (D)</p>
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What is the purpose of the Viterbi algorithm in hidden Markov models?

<p>To find the most likely state sequence given the observation sequence (A)</p>
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Which of the following is a key difference between the forward algorithm and the Viterbi algorithm in hidden Markov models?

<p>The forward algorithm computes the likelihood, while the Viterbi algorithm finds the most likely state sequence (C)</p>
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What is the role of the observation probability $P(x_t|q_t)$ in the computation of the likelihood of the observation sequence in a hidden Markov model?

<p>It represents the probability of observing $x_t$ given that the current state is $q_t$ (B)</p>
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What is the role of the transition probability $P(q_t|q_{t-1})$ in the computation of the likelihood of the observation sequence in a hidden Markov model?

<p>It represents the probability of transitioning from state $q_{t-1}$ to state $q_t$ (C)</p>
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What is the purpose of the initial state probability $P(q_1)$ in the computation of the likelihood of the observation sequence in a hidden Markov model?

<p>It represents the probability of the initial state $q_1 (C)</p>
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