Jamas v. Krpan
In Jamas v. Krpan, 116 Ariz. 216, 568 P.2d 1114 (App. 1977) the Court explained when expert testimony was properly excluded because it invaded the province of the jury.
Jamas was a malpractice case where the expert was permitted to opine that the defendant's conduct was negligent, but was not permitted to testify that the conduct amounted to gross negligence. Id. at 217, 568 P.2d at 1115.
On appeal, the court concluded that:
Although a jury may not be competent to determine medical malpractice without the aid of expert testimony that the physician had deviated from the accepted standard of care, it does not necessarily follow that the jury, having been informed of community standards, is incompetent to judge the nature or gravity of the deviation. . . . Id.
Thus, the expert's opinion was both unhelpful to the jury and expressed a legal conclusion that was the jury's responsibility.