On October 1, 2026, Paul presented “Expert Reports: Selection and Strategy” at Osgoode Professional Development’s Anatomy of a Medical Malpractice Case, a one-day live online program chaired by Richard M. Bogoroch of Bogoroch & Associates LLP and Bronwyn Martin of Moodie Mair Walker Lawyers. The program walked a medical malpractice action from end to end, working from a case drawn from a real-life fact scenario: the case summary and legal issues, pleadings, audit trails and unauthorized access to medical records, discovery, expert evidence, and a closing judicial panel with Justice Graeme Mew. A replay of the program is scheduled for December 4, 2026.
Paul’s session sat at the centre of the afternoon, after discovery and before the sessions on stress-testing expert evidence for trial and the late delivery of expert reports. Its premise was simple. In most civil litigation, expert evidence supports the case. In medical negligence, it very largely is the case.
Why the Expert Is the Case
A physician’s standard of care cannot ordinarily be established without expert evidence. Where the subject matter is technical, the Supreme Court of Canada has been explicit that a trier of fact generally cannot condemn a practice the profession itself accepts: ter Neuzen v Korn, 1995 CanLII 72 (SCC), [1995] 3 SCR 674. Causation is decided not on scientific certainty but on inference drawn from opinion evidence that is rarely unanimous: Snell v Farrell, 1990 CanLII 70 (SCC), [1990] 2 SCR 311. And in a catastrophic injury case, quantum is built almost entirely out of assumptions supplied by experts.
The practical consequence is that expert selection and report quality are outcome determinative. A weak report invites a trial the plaintiff may lose. A strong report significantly increases the likelihood of settlement, and of success at trial if it comes to that. Choosing the right expert is within counsel’s control, and it has to be done early.
The paper was organized around three questions: choosing the right expert, ensuring credibility and independence, and crafting reports that withstand scrutiny. Each section closed with a block setting out how the defence will approach the same ground.
Three Jobs, Three Experts
Medical negligence litigation asks experts to do three distinct things, and the temptation to ask one witness to do more than one of them should usually be resisted.
The standard of care expert describes what a reasonable practitioner in similar circumstances would have done, judged as at the time and without the benefit of hindsight. The most recent word from the Supreme Court is Armstrong v Ward, 2021 SCC 1, [2021] 1 SCR 3, which restored a trial finding of breach and cautioned against reasoning backwards from a bad outcome. The expert’s task is to describe the reasonable step, not to work backwards and find blame.
Causation is where files are most often lost, and usually because counsel asked the wrong question. The expert does not need to be certain. Snell permits a robust, common sense inference of causation without positive scientific proof. Clements v Clements, 2012 SCC 32, [2012] 2 SCR 181, confirms that the but for test is the default and that material contribution to risk is exceptional. Athey v Leonati, 1996 CanLII 183 (SCC), [1996] 3 SCR 458, holds that the defendant’s negligence need only be a cause that materially contributes to the injury, not the sole cause. Ediger v Johnston, 2013 SCC 18, [2013] 2 SCR 98, reaffirms Snell while making clear that the robust inference is not available where there is contrary evidence. And in delayed diagnosis cases involving several defendants, Sacks v Ross, 2017 ONCA 773, holds that the but for test continues to apply to each defendant.
Damages experts (life care planners, occupational therapists, economists, actuaries, vocational consultants and forensic accountants) produce reports of a different character. They are assumption driven, and they are usually attacked on their assumptions rather than their qualifications.
Two traps follow. The first is the single point of failure: a plaintiff who asks one witness to carry both standard of care and causation gives the defence one target, and an expert discredited on causation tends to be discredited on standard of care as well. The second is the certainty trap. An expert instructed in terms of medical certainty rather than the balance of probabilities will either overclaim, which can be fatal on cross-examination, or concede uncertainty in language a jury hears as doubt.
Choosing the Right Expert
The threshold criteria for expert evidence are familiar from R v Mohan, 1994 CanLII 80 (SCC), [1994] 2 SCR 9: relevance, necessity, the absence of any exclusionary rule, and a properly qualified expert. In medical malpractice practice, the fourth limb is where the real fighting happens.
The closer the expert’s specialty and practice setting to the defendant’s, the harder the opinion is to discount. A mismatch is usually argued as a matter of weight rather than admissibility, but that should not be mistaken for a safe harbour. Nursing standards require a nursing expert, and institutional or systems failures require an administration or systems expert. An expert who opines beyond their expertise fails the threshold on that issue, and the damage rarely stays contained. Conversely, an expert who will say that a question falls outside their field is worth more than one who will answer everything.
The Ontario reality makes all of this harder. Specialty communities here are small, and experts are often unwilling to opine against local colleagues. One answer is to look further afield, and out-of-province, American and British experts however with foreign experts, admissibility may be an issue. The trap is that a physician who measures the defendant against American practice norms is using the wrong yardstick. The legal standard is the Ontario standard, and the imported expert who has never worked within its constraints can become the softest target in the plaintiff’s case. Paul has written about one example, a jaw surgery claim in which the plaintiff’s American expert could not speak to the Canadian standard.
The paper set out a ten-point due diligence list to complete before any retainer, including conflicts, prior opinions and testimony on the same issue, the expert’s publication trail, prior judicial criticism, the balance of plaintiff and defence work, and a fee arrangement documented in writing and never contingent on outcome. It also distinguished the screening expert, who assesses merit before a claim is issued, from the testifying expert, who serves a Rule 53.03 report, signs the Form 53 acknowledgment, and is exposed to cross-examination. Whether one person should do both is a decision to make deliberately, case by case.
Credibility and Independence After White Burgess
Admissibility proceeds in two stages: the Mohan threshold, as structured by the Court of Appeal in R v Abbey, 2009 ONCA 624, and then a residual discretion that weighs probative value against prejudice, distraction, trial time and the risk of undue deference. White Burgess Langille Inman v Abbott and Haliburton Co, 2015 SCC 23, [2015] 2 SCR 182, folded independence and impartiality into the threshold. The question is whether the expert is unable or unwilling to give fair, objective and non-partisan evidence. Once a realistic concern is raised, the party tendering the expert must establish that the expert is able and willing. Exclusion is reserved for very clear cases, and the mere appearance of a relationship is not enough. A direct financial interest in the outcome is a different matter.
Bruff-Murphy v Gunawardena, 2017 ONCA 502, shows that the gate never closes. A defence psychiatric expert was qualified over the trial judge’s reservations, then became an advocate in the witness box, attacking the plaintiff’s credibility well beyond the opinion he was qualified to give. The Court of Appeal held that the trial judge’s gatekeeping role continues throughout the evidence and ordered a new trial. For the party who tendered the expert, the lesson is that a witness’s conduct on the stand can cost the verdict long after the qualification ruling went their way.
Paul closed this part with the behaviours courts have actually identified in partisan experts, offered as a pre-service audit of one’s own witness: refusing to engage with the opposing party’s account, adopting the retaining party’s information uncritically, arguing rather than answering, writing in counsel’s voice, and conceding nothing, including the obvious. That last behaviour is the one juries detect fastest and forgive least. Paul’s earlier article on expert witness qualification at trial for The Litigator covers related ground.
Crafting Reports That Withstand Scrutiny
Rule 53.03(2.1) of the Rules of Civil Procedure prescribes what an expert report must contain, and Paul’s recommendation was to treat its six requirements as the report’s table of contents rather than a compliance formality. Give the expert the subrule itself, not a summary, and ask for headings that track it. Subrule (e), which requires a summary of any range of opinion and the reasons for the expert’s position within it, is the weak link. In medical negligence there is almost always a range of responsible opinion, and an expert who identifies the competing view and explains why they reject it is far more persuasive than one who writes as though it does not exist.
Rule 4.1.01 makes the expert’s duty to the court paramount over any obligation to the party who retained them. Practitioners should also note that amendments to the Rules that came into force on September 1, 2026 revised Form 53 to add a certification that quotations in the report accurately reproduce their sources.
On the relationship between counsel and expert, Moore v Getahun, 2015 ONCA 55, leave to appeal to the Supreme Court of Canada refused, is the governing authority and is often remembered as deciding the opposite of what it decided. The Court of Appeal held that consultation and collaboration between counsel and the expert are not merely permissible but essential, and that drafts and related communications are protected by litigation privilege absent a factual foundation for a reasonable suspicion of improper influence. Counsel may explain the expert’s duty, the issues and the legal test, and may review a draft for clarity, completeness and compliance. Counsel may not shape the substance of the opinion, script answers, or coach a change in a genuinely held view. Because the threshold for piercing privilege is not a high wall, the prudent course is to conduct the relationship as though the file may one day be read.
Four disciplines make the difference in practice:
- Send the primary records, not a summary. An expert who works from counsel’s chronology has adopted counsel’s version of the facts.
- Write the instruction letter for the cross-examiner. It is disclosable under Rule 53.03(2.1)(c), so frame the question neutrally and direct the expert to the Ontario standard at the material time.
- Frame comments on a draft as questions and requirements, not edits. Asking whether the record supports an assumption is safe; rewriting the conclusion is not.
- Have the conversation early. The most common cause of a non-compliant report is an expert who was never told what the rule requires.
Timing matters as much as content. The session that followed Paul’s addressed the late delivery of expert reports, a subject Paul examined in his comment on late expert reports and Rule 53.08.
How the Defence Will Approach It
Each part of the paper ended with the defence playbook, because plaintiff counsel who know where the attack is coming from can build the case to withstand it. On qualification, the defence will bring the voir dire because it is cheap and need not succeed to be worth running, will probe the gap between a tertiary academic centre and a community hospital overnight, and will ask when the witness last actually did the work. On independence, defence counsel will read before they cross-examine: the expert’s own publications, prior testimony, and the ratio of plaintiff to defence work. On the report, the most productive single line is often the chronology question, because an expert who relied on counsel’s summary rather than the chart has a foundation problem that shows on the face of the report.
None of this runs one way. Every question about retainer history and medico-legal income can be asked of a defence expert, and CMPA-funded witnesses often have a longer and more concentrated retainer history than their plaintiff-side counterparts. Paul’s advice to both sides was the same: know your own expert’s numbers before your opponent puts them to the witness.
Looking Ahead
Expert evidence is a central target of Ontario’s ongoing Civil Rules Review. The final policy proposals include presumptive joint experts on economic loss and cost of care, expert conferencing without counsel in trial track cases, and tighter deadlines for serving reports. These are proposals, not rules in force, and their final form is not yet known. If adopted, they would change the mechanics of expert retention considerably, but the core disciplines discussed in the paper (the right expert, a genuinely independent opinion, and a report that shows its reasoning) would matter just as much.
What This Means for Patients and Families
For a patient or family considering a medical malpractice claim, this explains why the early stages of a case can take time and cost money before a claim is even issued. A responsible lawyer will usually obtain an independent medical opinion on both the standard of care and causation before advising whether a case can proceed, and will choose experts whose specialty and practice setting match the care in question. It also explains why some cases are declined: without supportive expert evidence, a malpractice claim generally cannot succeed. Paul has written more about how medical malpractice cases are evaluated in Ontario.
Speak With Paul Cahill
Paul Cahill is a partner at Davidson Cahill Morrison LLP and is certified by the Law Society of Ontario as a Specialist in Civil Litigation. He represents patients and families in medical malpractice claims across Ontario and works with referring lawyers on expert evidence and trial strategy. Every case depends on its own facts and expert evidence. If you have questions about a potential claim, contact Paul for a confidential consultation.
