The Gazette 1983

GAZETTE

NOVEMBER 1983

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

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'.

Stress Strain

Therefore, Stress Therefore, Load Area Therefore, Extension

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