Conditional Probability

probability of A

0.5

probability of B

0.5

●

In elementary probability, the conditional probability of an event

A

given an event

B

is the ratio

P(A⋂B)/P(B)

, provided

P(B)>0

. Visualize the events as plane regions (rectangles here) and their probabilities as the areas of those regions. Then the conditional probability

P(A|B)

is the fraction of

B

's area covered by the overlap of the two regions (purple). Use the sliders to adjust the probabilities (areas) of

A

and

B

and the black locator dots to change the location of

A

and

B

, thereby changing the overlap.