Consider a binary classification problem with two classes, A and B with prior probability P(A)=0.6𝑃(𝐴)=0.6 and P(B)=0.4𝑃(𝐵)=0.4 .Let X be a single binary feature that can take values 00 or 11. Given: P(X=1|A)=0.8𝑃(𝑋=1|𝐴)=0.8 and P(X=0|B)=0.7𝑃(𝑋=0|𝐵)=0.7. Determine which class the classifier will classify when X=1𝑋=1.
Question
Consider a binary classification problem with two classes, A and B with prior probability P(A)=0.6𝑃(𝐴)=0.6 and P(B)=0.4𝑃(𝐵)=0.4 .Let X be a single binary feature that can take values 00 or 11. Given: P(X=1|A)=0.8𝑃(𝑋=1|𝐴)=0.8 and P(X=0|B)=0.7𝑃(𝑋=0|𝐵)=0.7. Determine which class the classifier will classify when X=1𝑋=1.
Solution 1
To determine which class the classifier will classify when X=1, we need to calculate the posterior probabilities P(A|X=1) and P(B|X=1) and compare them. The class with the higher posterior probability will be the predicted class.
- Calculate P(X=1), the total probability that X=1:
P(X=1) = P(X=1|A)P(A) + P(X=1|B)P(B)
We know P(X=1|A) = 0.8 and P(A) = 0.6. We don't know P(X=1|B), but we know P(X=0|B) = 0.7, so P(X=1|
Solution 2
To determine which class the classifier will classify when X=1, we need to calculate the posterior probabilities P(A|X=1) and P(B|X=1) and compare them. The class with the higher posterior probability will be the predicted class.
The posterior probability can be calculated using Bayes' theorem:
P(A|X=1) = P(X=1|A) * P(A) / P(X=1) P(B|X=1) = P(X=1|B) * P(B) / P(X=1)
We know that P(X=1|A) = 0.8 and P(A) = 0.6. However, we don't know P(X=1) and P(X=1|B).
We can calculate P(X=1) using the law of total probability:
P(X=1) = P(X=1|A) * P(A) + P(X=1|B) * P(B)
But we don't know P(X=1|B). We only know that P(X=0|B) = 0.7. Since X is a binary feature, it can only take values 0 or 1. Therefore, P(X=1|B) = 1 - P(X=0|B) = 1 - 0.7 = 0.3.
Now we can calculate P(X=1):
P(X=1) = 0.8 * 0.6 + 0.3 * 0.4 = 0.48 + 0.12 = 0.6
Now we can calculate the posterior probabilities:
P(A|X=1) = 0.8 * 0.6 / 0.6 = 0.8 P(B|X=1) = 0.3 * 0.4 / 0.6 = 0.2
Since P(A|X=1) > P(B|X=1), the classifier will classify X=1 as class A.
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