Mass. Gen. Laws ch. 231 § 132
No new trial shall be granted in any civil action or proceeding on the ground of improper admission or rejection of evidence, or for any error as to any matter of pleading or procedure, if the judge who presided at the trial when application is made by motion for a new trial, or the appeals court or, the supreme judicial court when application is made by appeal or otherwise, deems that the error complained of has not injuriously affected the substantial rights of the parties; and, if it appears to such court that said error affects part only of the matter in controversy or some or one only of the parties, the court may direct final judgment as to part thereof, or some or one only of the parties, and may direct a new trial as to the other part only or as to the other parties.
Mass. Gen. Laws ch. 231, § 132