Qualified Expert Evidence
Expert opinion is testimonial evidence that gives an opinion on facts perceived by him or another that concerns an issue that is likely outside the experience and knowledge of the trier-of-fact (i.e. a lay person).  The expert must have special knowledge in the matter  and the opinion must be reasonably necessary to assist the trier-of-fact to make a proper judgment.  There must also be no exclusionary rules that would otherwise prohibit the evidence from being given. 
- Burden of Proof
The presumption for any given witness is that an opinion evidence is not admissible.
- Specialized Knowledge Vs Expert Opinion
A person testifying to specialized to technical knowledge will not necessarily be required to be qualified as an expert. Where they testify to their "factual knowledge" based on their "knowledge, observations and experience".
It has been accepted that technical evidence describing the "general rule and its exceptions" of the functioning of complex systems is not opinion evidence where the "understand[ing] the scientific and technical underpinnings" are not necessary to give reliable descriptions.
An expert who testifies to direct observation without opinion is not subject to the opinion rule of exclusion. This evidence is admitted in the same way as eye-witness evidence.
The expert is not to testify to facts, but rather only opinion to allow the trier-of-fact to draw inferences. 
Relevance is "a threshold requirement" to admitting expert evidence.
The key requirement for expert evidence is that the expert "possesses special knowledge and experience going beyond that of the trier of fact". As long as the court is satisfied that the witness is sufficiently experienced he should be qualified.
Any deficiencies of the knowledge or opinion simply goes to weight.
The means by which the skill, expertise, or knowledge was acquired goes to weight and not admissibility. It is acceptable to obtain the ability to give an opinion from study, instruction, practical experience or observations. Formal study in not necessary.
- Jury Instruction
Where a qualified expert gives opinion evidence outside of their field, the problem can generally be remedied through a "remedial instruction advising the jury to disabuse their minds of the inadmissible evidence".
- Standard of Review
Trial judges are entitled to "wide discretion" and deference when weighing the costs and benefits of admitting expert evidence.
Whether someone can be qualified as an expert is a question of law and is reviewable on a standard of correctness.
Folkes v Chadd (1782), 3 Dougl. 157
R v J-L J, 2000 SCC 51 (CanLII),  2 SCR 600, per Binnie J, at para 56
R v Abbey, 1982 CanLII 25 (SCC),  2 SCR 24, per Dickson J
R v Mohan, 1994 CanLII 80 (SCC),  2 SCR 9, per Sopinka J at 413 (the opinion must "provide information ‘which is likely to be outside the experience or knowledge of’” the trier of fact")
R v Bunniss, 1964 CanLII 673 (BC SC), (1964), 44 CR 262 (BC Co. Ct.), per Tyrwhitt-Drake J (an expert is "one who has by dint of training and practice, acquired a good knowledge of the science or art concerning which his opinion is sought, and the practical ability to use his judgment in that science")
- R v Terceira, 1998 CanLII 2174 (ON CA), 123 CCC 1 (Ont. CA), per Finlayson JA Mohan, supra, at p. 414 (the witness must be “shown to have acquired special or peculiar knowledge through study or experience in respect of the matters on which he or she undertakes to testify”)
R v Lovie, 1995 CanLII 801 (ON CA), per Finlayson JA
R v NO, 2009 ABCA 75 (CanLII), per curiam, at para 19
- Abbott and Haliburton Company v WBLI Chartered Accountants, 2013 NSCA 66 (CanLII), per MacDonald CJ (dissenting on other issue), at para 24
R v Hamilton, 2011 ONCA 399 (CanLII), 271 CCC (3d) 208, per curiam, at paras 273 to 284 - evidence from phone company as to the mechanical workings of cell towers and their relationship to the cell phone
R v Ranger, 2010 ONCA 759 (CanLII), OJ No 4840, per curiam -- cell phone tower evidence
contra R v Korski, 2009 MBCA 37 (CanLII), 244 CCC (3d) 452, per Steel JA -- required expert to testify on cell tower evidence
R v Potter, 2020 NSCA 9 (CanLII), per curiam, at paras 441 to 423
R v Ajise, 2018 ONCA 494 (CanLII), 361 CCC (3d) 384, at para 23
R v MacDonald, 2020 NSCA 69 (CanLII), at para 58, per Derrick JA (“ Technical evidence grounded in experience, without more, does not constitute expert evidence.”)
- Hamilton, supra, at paras 273, 274, 277
R v KA, 1999 CanLII 3793 (ON CA), 137 CCC (3d) 225, per Charron JA, at para 72
R v Parrott, 2001 SCC 3 (CanLII),  1 SCR 178, per Binnie J
R v Lavallee, 1990 CanLII 95 (SCC),  1 SCR 852, per Wilson J
- Mohan, supra, at p. 411
R v Béland, 1987 CanLII 27 (SCC),  2 SCR 398, per McIntyre J, at p. 415
R v Marquard, 1993 CanLII 37 (SCC),  4 SCR 223, per McLachlin J, at para 35
R v Chan, 1993 ABCA 383 (CanLII), 145 A.R. 304, per curiam, at para 9
- Chan, ibid., at para 9
- Chan, ibid., at para 9
Chan, ibid., at para 9 citing Sopinka, Lederman and Bryant, The Law of Evidence of Canada (1992), at pp. 536‑537: ("As long as the court is satisfied that the witness is sufficiently experienced in the subject matter at issue, the court will not be concerned with whether his or her skill was derived from specific studies or by practical training, although that may affect the weight to be given to the evidence.")
R v Rayner, 2000 NSCA 143 (CanLII), , 189 NSR (2d) 144, per Saunders JA
R v Melaragni, 1992 CanLII 12764 (ONSC), 73 CCC (3d) 348 (Ont. Ct. Gen. Div.), per Moldaver JA
R v NO, 2009 ABCA 75 (CanLII), 186 CRR (2d) 60, per curiam, at para 22
R v Sekhon, 2014 SCC 15 (CanLII),  1 SCR 272, per Moldaver J, at para 48
R v Clark, 2016 ABCA 72 (CanLII), per curiam, at paras 59 to 62
R v DD, 2000 SCC 43 (CanLII),  2 SCR 275, per Major J, at para 13
R v Soni, 2016 ABCA 231 (CanLII), 339 CCC (3d) 294, per curiam (2:1), at para 8
R v Dominic, 2016 ABCA 114 (CanLII), 335 CCC (3d) 178, per curiam, at para 17
R v Abbey, 2009 ONCA 624 (CanLII), 246 CCC (3d) 301, per Doherty JA, at para 97, leave to appeal to SCC refused
R v Bear (C.W.), 2013 MBCA 96 (CanLII), 299 Man R (2d) 175, per Steel JA, at para 81
Requirements of Expert Evidence ("Mohan" Test)
Example Fields of Qualifications
Opinion Evidence Outside of Qualification
Where a qualified expert testifies to matters directly outside of their area of qualification but is clearly within their area of special knowledge, the evidence can still be accepted absent any objection to the evidence by the opposing side. Likewise, a technical failure to qualify someone who clearly has expertise in the area, absent any objections, should be permitted to give expert evidence.
Where Technical Evidence Does Not Require Expert Evidence
There is some difference in opinion of whether or not expert qualification is needed to present evidence from cell phone towers. It is generally accepted that even where it is not necessary, it is useful to explain some of the records.
Evidence on the regular functioning of social media websites can in circumstances be accepted without qualification.
R v Hamilton, 2011 ONCA 399 (CanLII), per curiam, at paras 273 to 284 - evidence from phone company as to the mechanical workings of cell towers and their relationship to the cell phone. However, they did not give evidence triangulating the location of the accused's phone.
R v Ranger, 2010 ONCA 759 (CanLII), per curiam -- cell phone tower evidence accepted without qualification for general location evidence
cf. R v Korski, 2009 MBCA 37 (CanLII), per Steel JA -- required expert to testify on cell tower evidence
e.g. R v McBean, 2011 ONSC 3125 (CanLII), per Fuerst J
- R v Soh, 2014 NBQB 20 (CanLII), per LaVigne J