best. However, the non scientific enterprise allows some erroneous processes
such as illogical reasoning, selective observation, and even illicit stereotypes or
prejudice to operate. This is one of the major ways in which courts can make
substantially erroneous findings of fact (with a consequent injustice) by
allowing this form of irrationality to affect the outcome.
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Calculation
In some cases after the initial probability is established additional observation
refines the probability. This can be illustrated from the data in the worked
example found in discussion of probability.
150
First, assume that in a country
the probability that any person has ‘lurgi’ (a fictitious disease) is 5 percent.
Thus if we select a person at random the best estimate we now have is that the
person has a 0.05 probability of having lurgi. Assume now that someone
develops a diagnostic test for lurgi, but it is not completely accurate. If the
person has the disease the evidence clearly shows that the probability of the
test indicating the presence of the disease is 0.90. If the person actually does
not have the disease the probability that the test indicates that the disease is
present is 0.15.
Assume now that a person X is selected at random and the diagnostic test is
performed. The test indicates that the disease is present. What is the
probability that the person actually has the disease? Bayes Theorem, which is
explained in later discussion of probability, enables us to work out the answer
to this question.
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Objective Probability
Introduction
Inference can be based on classical probability, which is also referred to as
objective probability, a priori probability or the frequentist view of probability.
It is properly used only in a situation where events happen randomly so that
every outcome is equally likely.
Nature of Classical Probability
Classical probability is used in “cases where there is a finite number of
possible outcomes each of which is assumed to be equally probable”.
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Classic examples are the probability of drawing a specific card out of a pack
of 52 cards (1/52), or the probability that a specific number will come up on
the throw of a dice (1/6).
___________________
149
Chapter 27 Irrationality
150
Chapter 9 Probability
151
Chapter 9 Probability
152
Robertson (1993) p 459
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