(the standard of proof in criminal cases), “glaringly improbable”
234
and so on,
to indicate the strength of our belief.
Numerical Formulas
More commonly, however, probability is measured with numbers. This has
two advantages: numbers can be compared, and numbers can be used to
calculate other related or derived probabilities. There is, however, an
underlying disadvantage, because measuring probability with numbers is not an
exact process when the numbers are just estimates, which is often the case.
Hence the mathematical processes used in deriving and comparing
probabilities can convey a false sense of certainty.
Three scales are commonly used when probability is measured with numbers,
although they amount to the same thing. These consist of decimals with a
range between nought and one, percentages (a range between nought and one
hundred) and fractions.
Decimals
Decimals utilise
a range between nought (0) and one (1), which is labelled a
decimal measure because it entails using the decimal point. A probability of 0
is that the event will certainly not occur. A probability of 1 is that the event will
definitely occur. If the event
will occur one (1) time in four (4) it has a
probability of 0.25.
Percentages
Probability can be measured by percentages. Here an event that will certainly
occur has a probability of 100%, and an event that will certainly not occur has
a probability of 0%. An event that occurs one time in four has a probability of
25%.
Fractions
Probability can be expressed as a fraction. In some cases there is a practical
advantage in using fractions.
One example is racing. For the ordinary punter it makes sense to say, for
example, that a racehorse has a one in four chance of winning. A bookmaker
indicates this probability by quoting the horse at odds of three to one, that is
3-1 (against). This says that the horse has three chances of losing compared to
one chance of winning.
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234
This is a ground on which an appellate court can overturn a finding of fact
by a lower court – see House v the King (1936) 55 CLR 499, 505, Devries v Australian
National Railways Commission (1993) 177 CLR 472, 479
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