2023 Wisconsin Statutes & Annotations › Chapter 901 - Evidence — general provisions. › 901.03 - Rulings on evidence.

901.03
Rulings on evidence.
(1)
Effect of erroneous ruling. Error may not be predicated upon a ruling which admits or excludes evidence unless a substantial right of the party is affected; and
(a)
Objection. In case the ruling is one admitting evidence, a timely objection or motion to strike appears of record, stating the specific ground of objection, if the specific ground was not apparent from the context; or
(b)
Offer of proof. In case the ruling is one excluding evidence, the substance of the evidence was made known to the judge by offer or was apparent from the context within which questions were asked.
(2)
Record of offer and ruling. The judge may add any other or further statement which shows the character of the evidence, the form in which it was offered, the objection made, and the ruling thereon. The judge may direct the making of an offer in question and answer form.
(3)
Hearing of jury. In jury cases, proceedings shall be conducted, to the extent practicable, so as to prevent inadmissible evidence from being suggested to the jury by any means, such as making statements or offers of proof or asking questions in the hearing of the jury.
(4)
Plain error. Nothing in this rule precludes taking notice of plain errors affecting substantial rights although they were not brought to the attention of the judge.
History:  Sup. Ct. Order, 59 Wis. 2d R1, R9 (1973); 1991 a. 32.
An offer of proof must be made as a necessary condition precedent to the review of any alleged error in the exclusion of evidence. Without an offer there is no way to determine whether the exclusion was prejudicial. State v. Moffett, 46 Wis. 2d 164, 174 N.W.2d 263.
In order for an error to be “plain error" it must be so fundamental that a new trial must be granted so as not to deny a basic constitutional right. State v. Vinson, 183 Wis. 2d 297, 515 N.W.2d 314 (Ct. App. 1994).
Not all constitutional errors are plain errors. Some may be harmless errors. The state has the burden of showing that an error is harmless beyond a reasonable doubt. State v. King, 205 Wis. 2d 81, 555 N.W.2d 174 (Ct. App. 1996), 95-3442.
When a defendant alleges that a prosecutor's statements constituted plain error, the test is whether, in the context of the entire record of the trial, the statements so infected the trial with unfairness as to make the resulting conviction a denial of due process. State v. Cameron, 2016 WI App 54, 370 Wis. 2d 661, 885 N.W.2d 611, 15-1088.
Under sub. (1), an erroneous evidentiary ruling is reversible only if a substantial right of a party is affected. That means that an error is harmless if the party benefitted by the error shows beyond a reasonable doubt that the error complained of did not contribute to the verdict obtained. State v. Monahan, 2018 WI 80, 383 Wis. 2d 100, 913 N.W.2d 894, 14-2187.
The court uses several non-exclusive factors to aid the court's application of the harmless error rule in the evidentiary context: 1) the frequency of the error; 2) the importance of the erroneously included or excluded evidence to the prosecution's or defense's case; 3) the presence or absence of evidence corroborating or contradicting the erroneously included or excluded evidence; 4) whether erroneously excluded evidence merely duplicates untainted evidence; 5) the nature of the defense; 6) the nature of the state's case; and 7) the overall strength of the state's case. State v. Monahan, 2018 WI 80, 383 Wis. 2d 100, 913 N.W.2d 894, 14-2187.