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GATE 2024 DA – Question 64

Artificial Intelligence · Reasoning under uncertainty: Bayesian networks and inference · 2 marks · Numerical answer

Given the following Bayesian Network consisting of four Bernoulli random variables and the associated conditional probability tables:

[Figure: a network with $U$ pointing to $V$ and $W$, and $V$ and $W$ both pointing to $Z$. $P(U = 0) = P(U = 1) = 0.5$. $P(V|U)$ is 0.5 and 0.5 for both values of $U$. $W$ copies $U$: $P(W = 1|U = 1) = 1$ and $P(W = 0|U = 0) = 1$. $P(Z = 1|V, W)$ is 0.5 for $(V, W) = (0, 0)$, 0 for $(0, 1)$, 0 for $(1, 0)$ and 0.5 for $(1, 1)$.]

The value of $P(U = 1, V = 1, W = 1, Z = 1) = $ ______ (rounded off to three decimal places).

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Correct answer: 0.115 to 0.135

Explanation

By the factorisation of the network, $P(U, V, W, Z) = P(U)P(V|U)P(W|U)P(Z|V, W)$. So $P(U = 1, V = 1, W = 1, Z = 1) = P(U = 1)P(V = 1|U = 1)P(W = 1|U = 1)P(Z = 1|V = 1, W = 1) = 0.5 \times 0.5 \times 1 \times 0.5 = 0.125$.