Expert testimony is admissible where the expert is qualified, the testimony is based on sufficient facts or data, is the product of reliable principles and methods, and reflects a reliable application of those methods to the facts.
Qualification by knowledge, skill, experience, training or education — subject-matter specific. A distinguished economist may be unqualified on an industry-specific damages question.
Reliability factors. Testability, peer review and publication, known or potential error rate, standards controlling the technique, and general acceptance. These are illustrative rather than a checklist, and flexible for experience-based fields.
Where commercial experts fail. Damages models that assume liability facts without support; lost profits projections for businesses without a track record; reliance on a single client-supplied assumption that drives the entire result; methodologies invented for the litigation; and failure to account for obvious alternative causes.
Analytical gap. A court may exclude where there is simply too great a gap between the data and the opinion offered — the expert’s ipse dixit is not a sufficient bridge.
The application prong has been emphasised in recent amendments: it is not enough that the method is reliable in the abstract; the expert must have applied it reliably here.
Disclosure. Reports must contain a complete statement of opinions and the basis; opinions outside the report are excluded.