If you don’t know where a line is, you can’t say whether someone has crossed it. That principle applies in spades to expert witnesses, particularly when their role in the case calls on them to help the jury understand where the lines are drawn in their field. Thus, for example, a toxicology expert who cannot say how much chemical exposure is required to produce a toxic effect cannot opine that the plaintiff’s exposure was sufficient to cause that effect. Likewise, an expert who criticizes a product’s warnings but cannot articulate an adequate warning does not pass muster under Federal Rule of Evidence 702. See, e.g., Bourelle v. Crown Equip. Corp., 220 F.3d 532, 538-39 (7th Cir. 2000).
The Seventh Circuit recently gave yet another illustration of this principle at work. In Burns v. Sherwin-Williams Co., — F.4th —, 2023 WL 5210857 (7th Cir. 2023), a plaintiff truck driver was injured while making a delivery at defendant’s store using a “walkie” (i.e., an electric forklift that the operator walks behind) owned by defendant. Plaintiff was backing the walkie down a ramp and toward a dumpster with pallets laying on the ground nearby. Unfortunately, he underestimated the walkie’s stopping distance. He tried “plugging” – i.e., pushing the thumb switch in the opposite direction to slow and then reverse the walkie – and then engaged the emergency brake, but it was too late. He trapped his foot against the pallets, then fell over and broke his ankle.