Expert evidence is an exception to the general rule barring opinion evidence. In R. v. Mohan,  2 S.C.R. 9, the Supreme Court of Canada set out the four criteria for the admissibility of expert evidence: 1) relevance, 2) necessity in assisting the trier of fact, 3) the absence of any exclusionary rule, and 4) proper qualification. The party tendering expert evidence has the burden to satisfy the four Mohan criteria on a balance of probabilities.
 In discussing the second criterion at pp. 23, 24 of Mohan, the Supreme Court referred to the concept of helpfulness to a trier of fact. The court concluded that the appropriate test for necessity is whether the expert is capable of assisting the trier by providing information likely to be beyond the trier’s knowledge and experience.
 In determining whether an expert’s evidence will be helpful, a court will, as a matter of common sense, look to the question of the expert’s independence or objectivity. A biased expert is unlikely to provide useful assistance.
 Courts have taken a pragmatic approach to the issue of the independence of expert witnesses. They have recognized and accepted that experts are called by one party in an adversarial proceeding and are generally paid by that party to prepare a report and to testify. The alignment of interest of an expert with the retaining party is not, in and of itself, a matter that will necessarily encroach upon the independence or objectivity of the expert’s evidence.
 That said, courts remain concerned that expert witnesses render opinions that are the product of their expertise and experience and, importantly, their independent analysis and assessment. Courts rely on expert witnesses to approach their tasks with objectivity and integrity. As Farley J. said in Bank of Montreal v. Citak,  O.J. No. 1096, “experts must be neutral and objective [and], to the extent they are not, they are not properly qualified to give expert opinions.”
 When courts have discussed the need for the independence of expert witnesses, they often have said that experts should not become advocates for the party or the positions of the party by whom they have been retained. It is not helpful to a court to have an expert simply parrot the position of the retaining client. Courts require more. The critical distinction is that the expert opinion should always be the result of the expert’s independent analysis and conclusion. While the opinion may support the client’s position, it should not be influenced as to form or content by the exigencies of the litigation or by pressure from the client. An expert’s report or evidence should not be a platform from which to argue the client’s case. As the trial judge in this case pointed out, “the fundamental principle in cases involving qualifications of experts is that the expert, although retained by the clients, assists the court.”
 The report of the Goudge Inquiry, Inquiry into Pediatric Forensic Pathology in Ontario (Toronto: Ontario Ministry of the Attorney General: 2008), at p. 503, noted the importance of expert witness independence, quoting the principles described by the Court of Appeal of England and Wales in R. v. Harris and others,  EWCA Crim 1980, at para. 271:
(1) Expert evidence presented to the court should be and seen to be the independent product of the expert uninfluenced as to form or content by the exigencies of litigation.
(2) An expert witness should provide independent assistance to the court by way of objective unbiased opinion in relation to matters within his expertise. An expert witness in the High Court should never assume the role of advocate.
 In most cases, the issue of whether an expert lacks independence or objectivity is addressed as a matter of weight to be attached to the expert’s evidence rather than as a matter of the admissibility. Typically, when such an attack is mounted, the court will admit the evidence and weigh it in light of the independence concerns. Generally, admitting the evidence will not only be the path of least resistance, but also accord with common sense and efficiency.
 That said, the court retains a residual discretion to exclude the evidence of a proposed expert witness when the court is satisfied that the evidence is so tainted by bias or partiality as to render it of minimal or no assistance. In reaching such a conclusion, a trial judge may take into account whether admitting the evidence would compromise the trial process by unduly protracting and complicating the proceeding: see R. v. Abbey, 2009 ONCA 624, 97 O.R. (3d) 330, at para. 91. If a trial judge determines that the probative value of the evidence is so diminished by the independence concerns, then he or she has a discretion to exclude the evidence.
 In considering the issue of whether to admit expert evidence in the face of concerns about independence, a trial judge may conduct a voir dire and have regard to any relevant matters that bear on the expert’s independence. These may include the expert’s report, the nature of the expert’s retainer, as well as materials and communications that form part of the process by which the expert formed the opinions that will be the basis of the proposed testimony: see R. v. INCO Ltd. (2006), 80 O.R. (3d) 594, at p. 607 (S.C.).
 An appellate court will accord deference to a trial judge’s decision to exclude evidence of an expert on the basis that the proposed evidence lacks independence. On reviewing such a decision, an appellate court will look to whether the trial judge applied the proper legal principles and whether the trial judge’s conclusion was supported by the evidence. Absent such an error, an appellate court will not interfere.
 I would not interfere with the trial judge’s decision in this case. The trial judge had regard to the appropriate legal principles and there was ample evidence to support her conclusion that Mr. Anson-Cartwright’s proposed evidence lacked independence.