Clearly the complementarity rule is relevant to proof of facts. The sum of each
version of the facts being true equals 100%. Put another way, the probability
to which the initiating party has proved their case is the complement of the
degree to which the responding party has proved their case. Consequently,
estimating the probability that any version of the facts is true is grading the
relative certainty of truth of other versions. As the probability of one version
rises, the probability other versions must fall –
this is how one version
complements the others.
To demonstrate how this applies, let us label the standard of proof for an
initiating party (plaintiff or prosecutor) as X%. If they do not make X%, they
lose their case and the defendant wins.
Now the essential proposition is that
there are only two possible outcomes - the plaintiff or prosecutor wins on the
one hand or the defendant wins on the other. There is no middle ground (as
there is, for example in Scotland with the verdict of ‘not proven’).
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All a
defendant has to do, therefore, is to rebut the case for the other side. This
means that they have to prove their case only to the extent that they prevent the
initiating party from proving theirs to the X% that is required. This means that
they must prove their case to the extent of (100 –
X + 1) which can be
simplified to (101-X)%.
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This relationship can be displayed in a table. For this table the party who
initiates litigation
such as a prosecutor, plaintiff or applicant is labelled
“Initiator” and the party defending the case such as the defendant, accused or
respondent is labelled “Responder”. Here is the diagram:
Initiator
0%
20%
40%
50%
60%
80%
100%
100%
80%
60%
50%
40%
20%
0%
Responder
Total
100%
100%
100%
100%
100%
100%
100%
Total
Figure 21.1 Onus and Standard of Proof
Step 3: Standard of Truth
At this stage the court has before it, because of Step 1, the versions of the
facts of the case alleged by each party and presented in their evidence. In Step
2 the court has assessed the probability that each overall version of the facts is
true. Now, in Step 3, the court determines whether this probability reaches the
legally required standard of proof.
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This is the standard of truth set by law
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The not proven verdict is often called the Scottish Verdict or the Scotch
Verdict. For a proposal to introduce the not proven verdict into the United States
see Bray (2005).
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Chapter 25 Standard of Truth
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There is fuller discussion of the rationale for deploying probability in the
task of fact finding in Christopher Enright Legal Reasoning Chapter 9 Probability
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