A common way of expressing probability is in the form
of the odds of an event. The odds of an event
is defined as the ratio of the probability of the event
occurring to the probability of it not occurring i.e.
. So an event with probability 0.001
has odds of 1/999 (or 999:1 against in gambling jargon).
Odds can range from zero to infinity and are equal to one
for events whose occurrence and non-occurence are equally
likely (known as evens by gamblers). Odds can be used
to assess the total risk of a set of independent events by
simply multiplying together the odds of the individual events.