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GATE DA 2025 Q35: Naive Bayes: probability of misclassifying x

Question

The naive Bayes classifier is used to solve a two-class classification problem with class-labels y1,y2y_1, y_2. Suppose the prior probabilities are P(y1)=13P(y_1) = \frac{1}{3} and P(y2)=23P(y_2) = \frac{2}{3}. Assuming a discrete feature space with

P(x∣y1)=34andP(x∣y2)=14P(x|y_1) = \frac{3}{4} \quad \text{and} \quad P(x|y_2) = \frac{1}{4}

for a specific feature vector xx. The probability of misclassifying xx is ____ (Round off to two decimal places)

Try it: numerical answer: type the value

Numerical answer. No negative marking.
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Answer: 0.39 to 0.41 (official GATE 2025 final answer key)

Concept used: Bayes decision rule: choose the class with the larger posterior; the error is the other posterior.

First step: Compare P(x∣y1)P(y1)=34⋅13P(x|y_1)P(y_1) = \frac{3}{4} \cdot \frac{1}{3} with P(x∣y2)P(y2)=14⋅23P(x|y_2)P(y_2) = \frac{1}{4} \cdot \frac{2}{3}, then normalise to get the posteriors.

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Source: official GATE 2025 DA question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key