it is still a road worth travelling because probable truth is better than no truth at
all.
Probability
While scientists aim for procedures and testing processes that are perfect they
cannot always achieve this in principle or in practice. For example, some
testing procedures regularly give a false positive (identifying the presence of X
when it is really absent) or a false negative (identifying the absence of X when
it is really present). In these cases the tests do not give absolute truth but a
probability of truth.
To illustrate this, assume that scientific evidence is based on an observed fact,
OF, a proposed scientific rule, SR, and a testing procedure, TP. If the validity
of each of the three processes are independent then the probability of a true
result, TR is the multiple of the probabilities that each of these three task yields
the truth. These individual probabilities are set out in the following table:
Probabilities
Symbols
Probability that the observed fact is true
P(OF)
Probability that the proposed scientific rule is correct
P(SR)
Probability that the testing procedure is reliable
P(TP)
Figure 5.15 Probabilities for Scientific Testing
Given these individual probabilities, the probability of a true result, P(TR) is
given by the following formula”
P(TR) = P (OF) x P(SR) x P(TP).
As is obvious, in real life the combination of these items may be more
complex than is the case in the example here. However the preceding analysis
indicates the essential character of the reasoning, which is a combination of
deductive certainty and probabilistic uncertainty.
Admissibility of Scientific Evidence
Introduction
Common law legal systems admit scientific evidence when there is sufficient
certainty that the evidence will be based on good science. Clearly any legal
system needs to devise rules to determine when alleged scientific evidence is
admissible and the reliance that can be placed on that evidence. Illustrative of
some of the attempts to formulate this rule are two stages of development of
such a rule in the United States.
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