From the Supreme Court of South Australia sitting en banc, a useful overview of the topic from the Australian perspective.

King CJ said,

The general rule is that a witness may give evidence only as to matters observed by him. His opinions are not admissible. One of the recognized exceptions to this rule is that which relates to the opinions of an expert. This exception is confined to subjects which are not, or are not wholly, within the knowledge and experience of ordinary persons;…

On such subjects a witness may be allowed to express opinions if the witness is shown to possess sufficient knowledge or experience in relation to the subject upon which the opinion is sought to render his opinion of assistance to the court.

Before allowing a witness to express such opinions, the judge must be satisfied that the witness possesses the necessary qualifications, whether those qualifications be acquired by study or experience or both. But when it is established that the witness is an expert in the relevant field of knowledge, he will be permitted to express his opinion, however unconvincing it might appear to be…., subject always, of course, in a criminal trial to the discretion to exclude evidence whose prejudicial effect is disproportionate to its probative value. The weight to be attached to his opinion is a question for the jury.

The principal argument advanced…for the appellant was that counsel at the trial should have been permitted on the voir dire to investigate by cross-examination the question of the adequacy of the material upon which the witness based his opinion. He argued that that line of cross-examination amounted to an investigation of the method by which the opinion was reached and that the Judge was required to be satisfied as to the soundness of the methodology adopted before allowing the evidence to be given.

Before admitting the opinion of a witness into evidence as expert testimony, the judge must consider and decide two questions.

The first is whether the subject matter of the opinion falls within the class of subjects upon which expert testimony is permissible. This first question may be divided into two parts:

(a) whether the subject matter of the opinion is such that a person without instruction or experience in the area of knowledge or human experience would be able to form a sound judgment on the matter without the assistance of witnesses possessing special knowledge or experience in the area, and

(b) whether the subject matter of the opinion forms part of a body of knowledge or experience which is sufficiently organized or recognized to be accepted as a reliable body of knowledge or experience, a special acquaintance with which by the witness would render his opinion of assistance to the court.

The second question is whether the witness has acquired by study or experience sufficient knowledge of the subject to render his opinion of value in resolving the issues before the court.

An investigation of the methods used by the witness in arriving at his opinion may be pertinent, in certain circumstances, to the answers to both the above questions. If the witness has made use of new or unfamiliar techniques or technology, the court may require to be satisfied that such techniques or technology have a sufficient scientific basis to render results arrived at by that means part of a field of knowledge which is a proper subject of expert evidence….

An investigation of the methods adopted by a witness may be relevant to an assessment of his qualifications as a witness if such an investigation might reveal that the witness has “posing as an expert made assertions that are contrary to proved scientific facts or to the known phenomena of nature, thus exposing his ignorance of the learning he professed”…, or that the witness has adopted methods which are so unscientific as to expose that ignorance.

Where the witness possesses the relevant formal qualifications to express an opinion on the subject, an investigation on the voir dire of his methods will rarely be permissible on the issue of his qualifications. There may be greater scope for such examination where the alleged qualifications depend upon experience or informal studies.

The trial judge must have a wide discretion as to what may be investigated on the voir dire and the scope for interference by an appellate court with the way in which the discretion is exercised must be limited.

Generally speaking, once the qualifications are established, the methodology will be relevant to the weight of the evidence and not to the competence of the witness to express an opinion. The suitability and adequacy of the methods used may well be themselves a matter of expert opinion.

There could be no question in the present case as to whether the subject matter was proper for expert testimony. Comparison of handwriting and identification of signatures is a well-established field for expert testimony.

Although there is no course of formal study in the subject, the qualifications of the witness acquired by informal study, practical instruction and experience are considerable. The question whether the available material was sufficient to form a sound opinion could not affect those qualifications and its answer must depend at least in part upon expert opinion.

In my opinion the learned Judge was correct in taking the view that questions on that topic related to weight and not to admissibility. He was therefore correct in preventing counsel from pursuing them on the voir dire. It should be pointed out that counsel did pursue the topic in general cross-examination. It would have been open to counsel to have asked the Judge, after that cross-examination, to reconsider his ruling as to the admissibility of the expert evidence, but she did not do so. In my opinion the evidence was admissible.

Edited for ease of reading. A portion of this judgment was cited in JP Morgan Chase (2016)