Another example involves the probability of drawing a card of a particular type
from a pack of 52. This probability is 1 in 52, that is 1/52. This can be
expressed as a percentage, ie 0.9230769, but this is a clumsy number and is
also an imperfect approximation. Moreover, most of us know that a deck has
52 cards, so fractions with fifty two (52) as the base or denominator make
immediate sense.
Illustration
To illustrate this, we can show the three means of measuring probability in a
table which portrays the probability of an event which has a one in four chance
of occurring:
Will Occur
Will Not Occur
Total
Percentage
25%
75%
100%
Decimal
0.25
0.75
1
Fraction
1/4
3/4
1
Figure 9.1 Measures of Probability
The total, 100% or 1, represents certainty. It is the sum of the probability that
an event will occur and will not occur. Since an event can only occur or not
occur, there is absolute certainty that there will be one or other outcome.
Use in Practice
All three measures are used in practice, and in any event, one can be converted
to the other. In this regard, percentage and decimal probabilities, as the table
above shows, are fundamentally the same, except that the denominator of the
fraction is one (1) for decimals and one hundred (100) for percentages.
Applying Probability
Introduction
Probability, as has been stated, accounts for uncertainty. In fact it covers
uncertainty in at least three areas – how likely is an event to occur, how often
will an event occur and how true is a proposition. Conveniently these areas are
separately treated since they emphasise different applications of probability.
Likelihood
Will an event happen or will it not? Probability can answer this question by
stating the likelihood that a particular event will happen or has happened. For
example if a person tosses a coin there is a 50% chance that the coin will come
down heads and 50% chance that it will come down tails.
Frequency
Probability may be used to state the number of expected occurrences of an
event when an experiment is repeated.
For example if we toss a coin 10,000
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