The Gazette 1983

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

and

DPP

-v- •

Kinlan

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

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

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

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