ABSTRACT
The major case, which sets out both how the expert acquires his knowledge and what is the best form of EVIDENCE, is English Exporters (London) Ltd. v Eldonwall Ltd. [1973] 1 Ch 415. The judgment is therefore quoted at length:
Let me ... confine myself to the ADMISSIBILITY of hearsay in chief and in re-examination in these valuation cases. In such circumstances, two of the heads under which the valuer's evidence may be ranged are opinion evidence and factual evidence. As an expert witness, the valuer is entitled to express his opinion on matters within his field of competence. In building up his opinions about values, he will no doubt have learned much from transactions in which he has himself been engaged, and of which he could give first-hand evidence. But he will also have learned much from many other sources, including much of which he could give no first-hand evidence. Textbooks, journals, reports of auctions and other dealings, and information obtained from his professional brethren and others, ... have contributed their share. Doubtless much, or most, of this will be accurate, though some will not, and even what is accurate so far as it goes may be incomplete, in that nothing may have been said of some special element which affects values. Nevertheless, the opinion that the expert expresses is none the worse because it is in part derived from matters of which he could give no direct evidence. Even if some of the extraneous information which he acquires in this way is inaccurate or incomplete, the errors and omissions will often tend to cancel each other out; and the valuer, after all, is an expert in this field, so that the less reliable the knowledge that he has about the details of some reported transaction, the more his experience will tell him that he should be ready to make some discount from the weight that he gives it in contributing to his overall sense of values. Some aberrant transactions may stand so far out of line that he will give them little or no weight. No question of giving HEARS A Y EVIDENCE in such cases; the witness states his opinion from his general experience.
