GAZETTE
NOVEMBER 1983
Stress
Strain
Therefore, Stress
Therefore, Load
Area
Therefore, Extension
a constant (called the
modulus of Elasticity E).
'E' x Strain
'E' x extension in length
original length.
Load x Original length
Area x 'E'.
Therefore, We have calculated with certainty what the
extension in length of the wire will be, and
this equation will be constant anywhere the
experiment is carried out in the same way,
even with different loads and wire lengths.
This law (taking Hooke's law for instance) may then
become part of the highest level of scientific statement we
can make. This is that it may become part of a "Theory".
A scientific theory is quite precise. It consists initially of a
scientific law in terms already explained. This immutable
law is taken as an axiom and provides the basis for further
experiments. If further laws, which we chooe to call
theorems, can be extracted from the basic axiom, then the
axiom itself, together with the theorems and their proofs
will collectively become a 'theory', e.g., Einstein's
"Theory of Relativity". Note that a 'theory' has a more
important place in the canons of scientific thought than
the mere 'law' from which it is derived.
It appears, therefore, that any concept of probability
less than certainty would be difficult for an expert giving
technical evidence. Since the languages of science and law
are not compatible we must proceed with caution. The
lawyer's familiarity with the concept of'probability' may
blind him to the totally different scientific concept. The
scientist likewise is unlikely to grasp the subtlety of the
word 'probability' having two different meanings when
put in the context of civil or criminal cases and both these
different to his own. Since justice itself rests on the
objective treatment of the facts it follows that the facts as
presented should be capable of being clearly understood
by the lawyers and the judge/jury.
The logical conclusion to be reached is that expert
evidence based on probability theories may have their
place in court but should not be treated as sacrosanct. A
full explanation of their basis in non-technical language
should be demanded.
Application of Probability Theory
Probability theory applied to any case may be
considered in two ways. The first is when scientific
probability is applied during expert evidence. This means
that probability theory is restricted to a narrow band of
evidence confined to the expert witness. Since this
probability theory rests on pure or abstract mathematics
its application to real life situations is always open to
challenge. Here one should differentiate between
scientific facts collected by the expert witness and not
open to question and the application of that fact to
activate a mathematical probability theory. For example,
the expert witness may say that a hair found on the
defendant's clothingwas similar to that of the victim and
not be contradicted. When the expert witness then
proceeds to show that the probability of this happening is
250
1 million to 1, he has overstepped from the abstract and
rigid world of science into the teeming, complex world of
reality. In short, his probability theory depends on
complete predictability and the events of the real world
contaminate this.
In expert evidence, the witness would be better advised
to confine his remarks to fact collection, and leave these
facts to the jury. In the given case, where the odds against
a hair of the same type as the deceased being found on the
defendant are stated as (say) 1 million to 1, then it is an
invitation to the defence to debunk the figures. It invites
argument following the contrast between probability
theories and real life situations. For instance, we know
that more people die of lung disease in Arizona because
those afflicted with this disease retire there to enjoy the
dry weather. We know the blood group of the majority of
Aran Islanders is shared more with those areas of
England from which Cromwell's invading force were
recruited rather than any Irish region. The application of
probability theory, even in the narrow context of expert
evidence can be too easily upset by a defence determined
to show the facts in the light of ihe statistician's short-
comings, i.e., with one foot in a fire and one foot in a
fridge then you are comfortable on average.
This could be seen in
People
-v-
Collins
1
and
DPP
-v- •
Kinlan
8
.
In both cases the errors in establishing the
probability of certain events together with attempts to
combine these probabilities was fatal to the cause. The
laboratory conditions of the pure probability theory will
never be available in real life.
The second case arises when probability theory is
applied on an even wider scale.
A fortiori,
the same
remarks apply to the idea promoted by Dr. Lindley? that
all
evidence presented in court should be expressed in
terms of probabilities and combined by means of a
"probability calculus". This silly idea rests on being able
to place a numerical value on the strength of each piece of
evidence and to combine these values mathematically to
give an overall verdict. The effect of this would be to play
Russian roulette with the defendant. It would reduce the
court hearing to the game of chance on which probability
theory is based. In addition, it would lead to defendants
without merit being discharged on appeal under the
weight of a rigorous attack on the mathematical analysis
of the prosecution. It is hard to agree with M. Stuart
10
who advocates "the wider use of probabilities and
statistical evidence in legal processes". It is significant
that there is no unanimity on the question of proof based
on probability. In 1977, A. Cohen" devoted his book
almost completely to the flaws in a theory of probability
related to court fact finding. A year later, Sir R.
Eggleston
12
wrote to claim that the reverse approach was
fundamental to fact finding. Perhaps they are both wrong
in looking at pure probability theory and could
compromise by adopting the scientific "hypothesis"
based on probability as shown in the diagram.
The first activity in the diagram is fact collection itself,
unaided by any theory of probability. These bare facts are
then scrutinised conceptually to give a hypothesis for
testing in terms of "If A occurs then B will probably
result". This is the basis of scientific analysis and readily
absorbs the concept of probability.
We can use the fact of
People
-v-
Collins
1
to illustrate
this. Here, a robbery was committed by an inter-racial
couple in a yellow c^r. He had a moustache and she was a
blond. On a statistical probability basis the prosecutor