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A capital P with two events separated by a vertical bar.
Also known as: Probability of A given B, Conditional probability symbol
P(A|B) is the probability of A given that B has happened. The vertical bar is read 'given', and reversing the two sides gives a different and often wildly different number — the base-rate error underlying most misread medical test results.
Denotes the probability of one event given that another has occurred; the vertical bar separates the event from the condition.
Contexts: Mathematics
Not symmetric. P(A|B) and P(B|A) are different quantities, and confusing them is the error that makes a positive result on an accurate test for a rare condition much less alarming than it first appears.
A capital letter P followed by round brackets containing two letters separated by a single vertical bar.
Both are core operators of probability notation.