(4)
Deduction. Scientific evidence given by an expert is based on
deduction. A common pattern is that the expert observes something linked to
the events. They carry out some form of scientific testing. This reveals some
additional fact, which, up to this point, was not obvious. In some cases the
link between the scientific testing and the additional fact can be purely
deductive. In other cases it may be deductive in a qualified way.
Stage 2: Overall Facts
Once the court has assessed the probability that specific pieces of facts are
true it then assembles the pieces that make up each overall version of the facts.
Having done this, the court seeks to assess the probability that each overall
version of the facts is true. To do this the court should deploy the rules of
probability.
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There are two aspects to assessing the probability that overall facts are true:
(i)
One Overall Version of the Facts. An overall version of the facts
constitutes a full account of the relevant events as contended by a party. It is
potentially sufficient, therefore, to establish their case. Generally this overall
version is based on an amalgamation of the specific facts given in evidence by
two or more witnesses. The court needs to estimate the probability that this
overall version of facts is true. This probability is derived mathematically from
the probability that specific facts within the overall version are true.
(ii)
Two or More Overall Versions. Generally a party’s case consists of just
one overall version. However, it can also be made up of two or more overall
versions of the facts.
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This happens there are two or more versions of one or
more of the material facts. Here the court has to assess the probability that
each overall version of the facts is true in order
to derive mathematically the
probability that a party has made out their case to the required degree of
certainty.
Complementarity Rule
The complementarity rule is a rule of probability. It states the axiomatic
proposition that the probability for all possible outcomes of an event add up
to 100%. So, if an event can either happen or not happen the probability of it
happening (for example 60%) and the probability of it non happening (40%)
total 100%.
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For a basic account how the rules of probability are deployed in fact
finding see Christopher Enright Legal Reasoning Chapter 9 Probability.
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With regard to two or more overall versions it is necessary to note that it is
always possible that there is a ‘missing’ version of the facts. Not all versions will
necessarily be raised in
court so a court has to take this possibility into account
when estimating probabilities that versions of the facts are true.
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