In elementary probability, the conditional probability of an event
given an event
is the ratio
, provided
. Visualize the events as plane regions (rectangles here) and their probabilities as the areas of those regions. Then the conditional probability
is the fraction of
's area covered by the overlap of the two regions (purple). Use the sliders to adjust the probabilities (areas) of
and
and the black locator dots to change the location of
and
, thereby changing the overlap.
