Conditional Probability: Two Children
A family has two children. You learn that at least one of them is a boy.
Answer the questions below and state your assumptions clearly.
Constraints & Assumptions
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Assume each child is independently a boy or girl with probability 1/2 unless you state otherwise.
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Treat birth order as relevant when enumerating equally likely outcomes.
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Pay attention to how the information is obtained.
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Distinguish "at least one is a boy" from "the older child is a boy" and from "a randomly selected child is a boy."
Clarifying Questions to Ask
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Are boys and girls equally likely and independent?
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Is the family sampled uniformly from all two-child families?
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How did we learn that at least one child is a boy?
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Are we conditioning on a statement about the family or on observing a randomly selected child?
Part 1 - At Least One Boy
Given only that at least one child is a boy, what is the probability that both children are boys?
What This Part Should Cover
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Enumerate ordered outcomes BB, BG, GB, and GG.
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Condition by removing GG.
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Compute the probability of BB among the remaining equally likely outcomes.
Part 2 - Older Child Is a Boy
How does the answer change if you are told that the older child is a boy?
What This Part Should Cover
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Condition on outcomes where the first child is B.
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Compare BB and BG.
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Explain why this conditioning information is more specific.
Part 3 - Randomly Selected Child Is a Boy
How does the answer change if a child is selected at random from the family and that child turns out to be a boy?
What This Part Should Cover
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Condition on the observation process, not only the family composition.
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Weight BB, BG, and GB by the probability that a randomly selected child is a boy.
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Explain why the answer differs from the generic "at least one boy" statement.
Follow-up Questions
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What if the statement is "at least one child is a boy born on Tuesday"?
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How would the answer change if boys and girls were not equally likely?
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Why do different observation mechanisms produce different answers?