Testing the Limits of Expert Testimony: Opining on Ultimate Issues After Webb v. Omni Block.

In Webb v. Omni Block, Inc., the Arizona Court of Appeals held expert witnesses cannot opine on the percentages of fault that may be assessed against multiple defendants.  In Webb, Defendant’s expert in a construction defect case opined that the supplier and various subcontractors were 35%, 30%, 25%, 10%, and 0% at fault for the failure of bond material holding the owner’s new home together.  The court held this testimony impermissibly instructed the jury how to decide the case in violation of Arizona Rule of Evidence 704(a), which allows expert opinions that embrace an ultimate issue unless the testimony directs, rather than assists, juries on ultimate issues.  Significantly, Rule 704 does not define “ultimate issue.”

The cases distinguishing between ‘assisting juries’ and ‘directing juries’ are inconsistent.  For example, in State v. Diaz, the Ninth Circuit held expert testimony permissibly assisted the jury when the expert testified a defendant’s prescriptions were written “outside the usual course of medical practice” and “without a legitimate purpose” in a case that required the State to prove the prescriptions were outside the usual course of professional practice and without a legitimate medical purpose.

But in in State v. Democker, the Arizona Court of Appeals held expert testimony impermissibly directed the jury in an issue regarding trust account fraud when the expert merely provided background information about the trust documents granting “great discretion to the trustee.”  Further, in Schuoler v. Napier, expert testimony impermissibly directed the jury when the expert opined on the defendant’s “reasonableness of holding a firearm at the time” in a case where the ultimate issue was if the defendant was justified in holding his firearm at the time.

It seems the only apparent consistency is inconsistency.