A trial judge must determine on a voir dire whether the individual is qualified as an expert, and if so, what the "nature and scope of the proposed expert evidence" will be. The qualification process is one of delineating the boundaries of the evidence and the language used.
The judge cannot permit the expert to give an opinion on common matters or matters that the expert has no special skills, knowledge, or training.
To qualify a person as an expert, the evidence must still meet the Mohan requirements:
1. the opinion must be relevant;
2. the opinion must be necessary to assist the trier-of-fact to draw the correct inference;
3. the absence of any other exclusionary rule;
4. the required qualifications of the proposed expert.
Additionally, under White Burgess the value of the evidence to be adduced must be such as to outweigh any confusion admitting the evidence might cause. That is:
1. the evidence would tend to usurp the duty of the trier of fact
2. the prejudicial effect outweighs the probative value
3. the time required outweighs its probative value
4. the cost required outweighs the probative value
5. the influence of the evidence outweighs the evidence's reliability.
Expert evidence must be impartial to be admissible. There must not be any bias or appearance of bias.
Mechanically before the evidence is heard the party seeking to adduce the evidence must "qualify" the expert. That is the voir dire – and the justice must say "now we are going into a voir dire". The evidence on the voir dire must show the expert fulfills the test above in which case the expert is allowed to testify.
The justice makes the ruling on admissibility – "I find XY is qualified to give evidence on AB". The voir dire is now complete and the trial continues.
The expert is called and testifies only on the matters allowed. None of the evidence on the voir dire is part of the trial so whatever is sought to be adduced must be adduced in the trial. The expert reports are not in evidence and most properly ought not to be put in evidence except on the express consent of all parties.
Of the Law Societies of Upper Canada and Nunavut