It is always important to remember that a single witness can provide both lay and expert testimony, and the line between the two is fine. Lay testimony is based upon one’s own observations. That testimony transforms to expert testimony if the witness brings his experience or training to bear on her “personal observations and makes connections for the jury based on that specialized knowledge.”
In Jones, a police officer walked – and crossed – this line when testifying about a dye pack that exploded in the messenger bag of a bank robber. He offered lay testimony when he described the characteristics of dye packs and situations in which he observed the effects of a dye pack detonating near someone’s skin. On the other hand, he offered expert testimony when he discussed where dye packs were manufactured, how the timers work, and the heat at which dye packs burn. Similarly, he would have offered expert testimony had the government asked him (which it did not) whether the burns on the bank robber’s leg “were of the type that would be caused by a dye pack exploding.”
A lawyer’s failure to distinguish lay from expert testimony can be significant. Most obviously, expert testimony is subject to certain legal safeguards and additional procedural rules before it can be admitted at trial, and a good trial lawyer will need to be on high alert when witnesses with specialized training – such as police officers – take the stand. The government’s error in Jones was slipping from lay to expert testimony without properly disclosing the expert testimony ahead of time, but the defendant’s failure to object at trial meant he received no relief on appeal.
Similarly, litigators need to be aware of this line at the summary judgment stage. A motion for summary judgment must be based on admissible evidence. Jones is a reminder to watch out for affidavits and deposition testimony from witnesses with specialized training or experience. Make sure that any such witness sticks to her own observations, and does not cross over into testimony that brings that specialized experience or training to bear on their observations, unless the witness and moving party have met the requirements of FRE 701 or 702.