ABSTRACT
In order to seek to establish the likelihood of the fragments of glass having come from the broken window, the prosecution called two expert witnesses who were principal scientific officers at the Home Office forensic laboratories with considerable experience in the analysis of fragments of glass, a Mr RA Cooke and a Mr KW Smalldon ... Mr Cooke explained that all glass has a refractive index, capable of being determined to five decimal places, which constitutes a measure as to how the light is bent when it passes into a particular piece of glass. He described the method of determining this index which he had used in this case. Using this method, he had compared several of the fragments of glass found in and on the shoes with each other and with the control sample, the glass of the broken window, and found that they all had the identical refractive index. Before expressing any opinion as to the likelihood or otherwise of the fragments of glass having come from the window, he was then asked about the frequency with which this particular refractive index is found to occur. In this connection he explained that it had been the practice of the Home Office Central Research Establishment to collate statistics of the refractive index of broken glass which had been analysed in forensic laboratories over a period of years, and that, having consulted these statistics, he found that this particular refractive index only occurred in 4% of all the analyses which had been made. If the analyses were limited to window glass, the frequency of occurrence was marginally lower. He was then asked whether, on the basis of his expert knowledge and the further analysis made by Mr Smalldon which he had seen and to which I turn in a moment, he was able to express any opinion as to the likely relationship of the glass fragments with the control sample. He answered this by saying: ‘Well, considering that only 4% of controlled glass samples actually have this refractive index I consider there is very strong evidence that the glass from the shoes is in fact the same as the glass from the window; in fact it originated from the window.’ Mr Smalldon then gave evidence that he had carried out a chemical analysis of the fragments of glass and of the control sample, and that he had found that ‘the two samples were similar on analysis and the analysis was typical of modern flat production glass’, ie window glass ... The point taken on this appeal was that the evidence of Mr Cooke, that the identical refractive index of the fragments of glass with that of the control sample occurred in only 4% of all controlled glass samples analysed and statistically collated in the Home Office Central Research Establishment, was inadmissible because it constituted hearsay evidence. It was said to be hearsay because Mr Cooke had no personal knowledge of the analyses whose results were collated in these statistics, save possibly a few for which he may have been personally responsible. This submission was challenged on behalf of the Crown, but no point was taken, in our view clearly rightly, on the ground that the admissibility of this evidence had not been challenged on behalf of the defence at the trial. In our view, the evidence was not inadmissible as hearsay. It is convenient to deal with this issue first on the basis of general principle and then to consider the authorities. Mr Cooke was admittedly an expert, and was giving evidence as an expert, on the likelihood or otherwise of the fragments of glass having come from the control sample, the broken window. As an expert in this field he was entitled to express an opinion on this question, subject to laying the foundation for his opinion and subject, of course, to his evidence being tested by cross-examination for evaluation by the jury. In the context of evidence given by experts it is no
more than a statement of the obvious that, in reaching their conclusion, they must be entitled to draw on material produced by others in the field in which their expertise lies. Indeed, it is part of their duty to consider any material which may be available in their field, and not to draw conclusions merely on the basis of their own experience, which is inevitably likely to be more limited than the general body of information which may be available to them. Further, when an expert has to consider the likelihood or unlikelihood of some occurrence or factual association in reaching his conclusion, as must often be necessary, the statistical results of the work of others in the same field must inevitably form an important ingredient in the cogency or probative value of his own conclusion in the particular case. Relative probabilities or improbabilities must frequently be an important factor in the evaluation of any expert opinion and, when any reliable statistical material is available which bears on this question, it must be part of the function and duty of the expert to take this into account. However, it is also inherent in the nature of any statistical information that it will result from the work of others in the same field, whether or not the expert in question will himself have contributed to the bank of information available on the particular topic on which he is called on to express his opinion. Indeed, to exclude reliance on such information on the ground that it is inadmissible under the hearsay rule might inevitably lead to the distortion or unreliability of the opinion which the expert presents for evaluation by a judge or jury. Thus, in the present case, the probative value or otherwise of the identity of the refractive index as between the fragments and the control sample could not be assessed without some further information about the frequency of its occurrence. If all glass of the type in question had the same refractive index, this evidence would have virtually no probative value whatever. The extent to which this refractive index is common or uncommon must therefore be something which an expert must be entitled to take into account, and indeed must take into account, before he can properly express an opinion about the likelihood or unlikelihood of the fragments of glass having come from the window in question. The cogency or otherwise of the expert’s conclusion on this point, in the light of, inter alia, the available statistical material against which this conclusion falls to be tested, must then be a matter for the jury. We therefore consider that Mr Cooke’s reliance on the statistical information collated by the Home Office Central Research Establishment, before arriving at his conclusion about the likely relationship between the fragments of glass and the control sample, was not only permissible in principle, but that it was an essential part of his function as an expert witness to take account of this material ... However, it was submitted that the present case was indistinguishable from the decision in Myers v DPP (1964) since Mr Cooke had not been personally responsible for the compilation of the Home Office statistics on which he relied, so that the inferences which he drew from them must be inadmissible because they were based on hearsay. In our view this conclusion does not follow, either as a matter of principle or on the basis of authority. We are here concerned with the cogency or otherwise of an opinion expressed by an expert in giving expert evidence. In that regard it seems to us that the process of taking account of information stemming from the work of others in the same field is an essential ingredient of the nature of expert evidence. So far as the question of principle is concerned, we have already explained our reasons for this conclusion. So far as the authorities are concerned, the position can be summarised as follows.
