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Chatty reference · stats1 week 8

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2026-08-16

28. Stats W8 — Conditional Probability

This is probably one of the highest-value Quiz 2 areas.

Concepts:

Definition:

P(A\mid B) = \frac{P(A\cap B)}{P(B)}

Multiplication:

P(A\cap B) = P(A\mid B)P(B)

Independence:

P(A\cap B)=P(A)P(B)

or equivalently:

P(A\mid B)=P(A)

when applicable.

Bayes:

P(A\mid B) = \frac{P(B\mid A)P(A)} {P(B)}

A question gives you:

evidence $B$

and asks:

probability of underlying condition $A$ .

That is often a Bayesian inversion problem.