Wisconsin standard for expert testimony: Unlike in the federal system, where the trial judge is a powerful gatekeeper with respect to the receipt of proffered expert evidence . . . . Wisconsin gives to the trial judge a more-limited role: the trial judge \u201cmerely require [s] the evidence to be \u2018an aid to the jury\u2019 or \u2018reliable enough to be probative.\u2019 \u201d State v. Walstad, 119 Wis.2d 483, 519, 351 N.W.2d 469, 487 (1984) (quoted source and one set of quotation marks omitted). Simply stated, this is a \u201crelevancy test.\u201d Ibid. Walstad explained:
\nIn a state such as Wisconsin, where substantially unlimited cross-examination is permitted, the underlying theory or principle on which admissibility is based can be attacked by cross-examination or by other types of impeachment. Whether a scientific witness whose testimony is relevant is believed is a question of credibility for the finder of fact, but it clearly is admissible<\/p>\n
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About deciding the case on public policy: “Although it is true, as Jones and The Innocence Network assert, that the supreme court has applied so-called \u201cpublic policy\u201d criteria to determine that certain types of evidence should never be received in Wisconsin courts, see State v. Dean, 103 Wis.2d 228, 278\u2013279, 307 N.W.2d 628, 653 (1981) (polygraph testing) (\u201c[W]e have not undertaken to evaluate the reliability of the polygraph. We recognize today … that the science and art of polygraphy have advanced and that the polygraph has a degree of validity and reliability. We are, nevertheless, not persuaded that the reliability of the polygraph is such as to permit unconditional admission of the evidence.\u201d); and Steele v. State, 97 Wis.2d 72, 97, 294 N.W.2d 2, 13\u201314 (1980) (excluding expert mind-science opinion whether defendant had criminal-intent capacity because such \u201cevidence is neither competent, relevant, nor probative for [that] purpose\u201d), those decisions are essentially islands in the law because the supreme court has declined to use public-policy criteria to truncate a trial court’s discretion in receiving or excluding evidence. See State v. Davis, 2002 WI 75, \u00b6\u00b6 14\u201315, 254 Wis.2d 1, 13\u201314, 645 N.W.2d 913, 919\u2013920. Indeed, as Morgan v. Krenke, 232 F.3d 562, 563\u2013564 (7th Cir.2000), recognized, Steele’s blanket exclusion was, in essence, based on the policy view that \u201c \u2018[w]hether or not there should be criminal responsibility is essentially a moral issue.\u2019 \u201d<\/p>\n
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Note: The Innocence Network submitted a brief requesting the rule: [T]hat in any future cases where ballistics opinion evidence is offered, the expert (1) may not be permitted to express an opinion that the field of ballistics, firearm and toolmark comparison identification is infallible, or has an error rate of zero, or any similar such pretension; (2) may not be permitted to express an opinion to a reasonable \u201cscientific\u201d or \u201cprofessional\u201d certainty; (3) may express only an opinion that it is \u201cmore likely than not\u201d that a particular identification is made; and (4) may only present opinion testimony to the jury if the witness is prepared to show (by photographs, video or other visual representations) what specific markings are being relied upon in reaching the opinion.<\/p>\n
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