In statistics, a theorem relating the
probability of particular events taking
place to the probability that events
conditional upon them have occurred. For
example, the probability of picking an ace at
random out of a pack of cards is 4/52. If two
cards are picked out, the probability of the
second card being an ace is conditional on
the first card: if the first card was an ace
the probability will be 3/51; if not it will
be 4/51. Bayes' theorem gives the probability
that given that the second card is an ace,
the first card is also.