What is the test for assessing harmlessness in a medical malpractice case?

"An assessment of harmlessness cannot include consideration of whether the witness' testimony would have been unchanged, or the jury's assessment unaltered ...; such an inquiry would obviously involve pure speculation, and harmlessness must therefore be determined on the basis of the remaining evidence." (Coy v. Iowa (1988) 487 U.S. 1012, 1021-1022, 108 S.Ct. 2798, 2803-2804, 101 L.Ed.2d 857.)