V. OPINION EVIDENCE. In general witnesses are allowed to testify only as to facts, and not as to their inferences or opinions based upon facts within their knowledge. To permit the witness to indulge in opinion testimony would be a usurpa tion of the function of the jury, whose duty it is to draw inferences of fact and to form an opin ion, where an opinion is necessary to the verdict. Thus the witness, when the facts of a conversa tion are in issue, must testify as to the terms of the conversation and not his conclusions as to its meaning. There are, however, three impor tant exceptions to the rule that opinion evidence is inadmissible. They are: (a) Matters of com mon experience; matters of common knowledge to a certain extent the result of inference. Thus, to testify that a certain day was cold, or that a knife was sharp. involves the operation of the witnesses mind in drawing a conclusion: but since these are matters of common experience about which the conclusions of the witness are as trustworthy as those of a jury. such testimony is legally admissible as evidence. (b) Matters not of common experience, but about which the opinion of the witness is under the circumstances more trustworthy than any which could he formed by the jury. Thus, a witness may be so situated with reference to an event or combina tion of circumstances as to be able to draw a more accurate conclusion from them than the jury, which should rely wholly upon verbal testi mony about the occurrence. Thus, the witness may
be allowed to give his opinion of the distance an anoroa(.1ibig street-ear and a pedes trian before warning of the approach was given; or, under certain circumstances, he may he al lowed to give his opinion of the rate of speed at which the car was moving. Ins presence at the lime of the event enables him to form a more accurate opinion than the jury. which can only rely upon a necessarily imperfect description of the occurrence. (c) Expert testimony. A wit ness linty be allowed to testify as to his opinion beeallse, by reason of experience or special study and investigation, he is better qualified to form an opinion than the jury. Thus, physicians, engi neers, handwriting experts, etc., are allowed to give opinion evidence in order to aid the jury in reaching a correct contention. They are not allowed, however, to express any opinion as to the truth or untruth of other evidence submitted to the jury, that being a matter of which the jury is qualified to judge. The testimony of experts, so fun' as it is opinion evidence, is based upon the evidence already before the jury, assuming it or parts of it to be true. For that reason questions asked of expert witnesses are usually required to be hypothetical in form.