The term "odds" is used to describe the likelihood
of an event occurring.
A person or
bookmaker or
casino will
quote the odds at which it is
prepared to accept a
bet from a
punter.
A
statistician will use the term
probability
for the same pupose.
Odds are commonly quoted using two different schemes.
Decimal and traditional.
Decimal odds are expressed as a single number being the
sum of a notional unit stake from the backer and the
corresonding stake from the layer. This single number
represents the total payout (i.e. including the
backer's stake) should the backer be successful. It is
the reciprocal of the probability.
Traditional odds are expressed as a ratio of the two
stakes. Usually the layer's stake is the first number
and the backer's the second. The word "on" is used
to reverse the order.
This is all best illustrated by example:
Probabilty 0.2 = Decimal odds 5 = Tradional 4 to 1
Probabilty 0.5 = Decimal odds 2 = Tradional 1 to 1
Probabilty 0.8 = Decimal odds 1.2 = Tradional 4 to 1 ON
Probabilty 0.4 = Decimal odds 2.5 = Tradional 3 to 2
Probabilty 0.01= Decimal odds 100 = Tradional 99 to 1