population the health system can allocate resources in advance to treat and
prevent it.
A statement of probability may go in either of two directions. It may use an
event to predict the probability of a second future event. This probability is
based on the behavioural law in the form “X causes Y”. Probability may also
be used to determine whether a past event might have happened. This can be
done, for example, when there is a causal law in the form “X is the sole cause
of Y”. When Y is observed to have happened, this causal law give us a basis
for believing that X happened, and happened at an earlier time.
Uncertainty with Probability
While probability came to prominence in the seventeenth century to explain
games of chance, it was not until 1933 that a major attempt was made to
explain it by postulating axioms on which probability properly should be built.
This was done by Andre Nilolaevich Kolmogorov (1903-1987) in his classical
text Foundations of the Theory of Probability. These axioms put probability
on a firm footing.
But notwithstanding this, the philosophical foundations of probability are still
subject to major debate. This uncertainty permeates discussion and literature
on probability. Yet for all this, the use of probability in the various sciences
continues unabated.
Given the purpose of this text it is not feasible to delve into any of the
philosophical debate on probability. Instead, discussion here is confined to an
attempt to explain in a clear a way as possible from a practical perspective
how and why probability is deployed in scientific inquiry.
Measuring Probability
Introduction
Probability, abbreviated ‘P’, ranges between absolute certainty that an event
will occur and absolute certainty that it will not occur. Probability can be
measured by verbal formulas or numerical formulas. Numerical formulas can
be decimals, percentages or fractions.
Verbal Formulas
Probability can be measured, in a rough sense, by verbal formulas. We can
and do use phrases such as “well founded,”
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“fairly certain,” “most likely,”
“with a strong chance,” “reasonably likely,” “beyond all reasonable doubt”
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Minister for Immigration v Guo (1996) 144 ALR 567 at 578
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