The measure comes before the math.

Since December 1, 2023, Federal Rule of Evidence 702 has said in its own text that the proponent must demonstrate to the court that it is more likely than not that each admissibility requirement is met, and that the opinion must reflect a reliable application of the method to the facts. The grounds that actually succeed against damages testimony are older than the amendment and legible on the face of a report: assumptions the expert relied on but never tested, a model that does not match the liability theory, no apportionment between lawful and unlawful causes, and a conclusion reaching past the data it rests on.

The but-for world is the counterfactual the damages number is measured against: what the plaintiff’s results would have been had the conduct not occurred. It is not a forecast, because it is tested against evidence from worlds that actually existed, and it is bounded by a legal standard that decides how speculative it may be.

Lost profits measure the earnings a business would have made and did not. Lost business value measures what the enterprise itself is worth against what it would have been worth. Courts treat them as distinct measures, and several have held that a plaintiff may not have both for the same injury, because the value of a business already reflects the profits it was expected to earn.

The calculation is a subtraction between two paths, and every term in it has a named public data source with a publication date. What separates a defensible earnings model from an attackable one is almost never the structure; it is the earnings base, the residual path, the benefit records, and whether the expert can say which table, which release, and which year.

A damages award arrives as one payment, so a stream of future losses has to be converted into a single present sum. The conversion runs on a rate, and the rate is not clerical: it carries the parties’ real disagreement about how certain the projected stream was, one state has abolished it for lost future earnings, another fixes it by statute, and in Jones & Laughlin Steel Corp. v. Pfeifer the Supreme Court of the United States declined to settle it.

Comcast Corp. v. Behrend, 569 U.S. 27 (2013), is cited for far more than it decided. It held that a model offered to show damages are measurable across a class must measure only the damages attributable to the one liability theory certified, and it said so on a record where the parties did not contest that class-wide measurability was required at all. Courts of appeals have since read it narrowly and consistently on that point; the live disagreement is elsewhere, over classes containing uninjured members.