Wis. Stat. § 907.03
The facts or data in the particular case upon which an expert bases an opinion or inference may be those perceived by or made known to the expert at or before the hearing. If of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject, the facts or data need not be admissible in evidence in order for the opinion or inference to be admitted. Facts or data that are otherwise inadmissible may not be disclosed to the jury by the proponent of the opinion or inference unless the court determines that their probative value in assisting the jury to evaluate the expert's opinion or inference substantially outweighs their prejudicial effect.
Wis. Stat. § 907.03
The trial court properly admitted the opinion of a qualified electrical engineer although he relied on a pamphlet objected to as inadmissible hearsay. E. D. Wesley Co. v. City of New Berlin, 62 Wis. 2d 668, 215 N.W.2d 657 (1974). A chiropractor could testify as to a patient's self-serving statements when those statements were used to form his medical opinion. Klingman v. Kruschke, 115 Wis. 2d 124, 339 N.W.2d 603 (Ct. App. 1983). The trial court erred by barring expert testimony on impaired future earning capacity based on government surveys. Brain v. Mann, 129 Wis. 2d 447, 385 N.W.2d 227 (Ct. App. 1986). While opinion evidence may be based upon hearsay, the underlying hearsay data may not be admitted unless it is otherwise admissible under a hearsay exception. State v. Weber, 174 Wis. 2d 98, 496 N.W.2d 762 (Ct. App. 1993). Although this section allows an expert to base an opinion on hearsay, it does not transform the testimony into admissible evidence. The court must determine when the underlying hearsay may reach the trier of fact through examination of the expert, with cautioning instructions, and when it must be excluded altogether. State v. Watson, 227 Wis. 2d 167, 595 N.W.2d 403 (1999), 95-1067. For a defendant to establish a constitutional right to the admissibility of proffered expert testimony, the defendant must satisfy a two-part inquiry determining whether the evidence is clearly central to the defense and the exclusion of the evidence is arbitrary and disproportionate to the purpose of the rule of exclusion, so that exclusion undermines fundamental elements of the defendant's defense. State v. St. George, 2002 WI 50, 252 Wis. 2d 499, 643 N.W.2d 277, 00-2830. This section implicitly recognizes that an expert's opinion may be based in part on the results of scientific tests or studies that are not his or her own. State v. Williams, 2002 WI 58, 253 Wis. 2d 99, 644 N.W.2d 919, 00-3065. Medical experts may rely on the reports and medical records of others in forming opinions that are within the scope of their own expertise. Enea v. Linn, 2002 WI App 185, 256 Wis. 2d 714, 650 N.W.2d 315, 01-2781. This section does not give license to the proponent of an expert to use the expert solely as a conduit for the hearsay opinions of others. As in a civil proceeding there is no independent right to confront and cross-examine expert witnesses under the state and federal constitutions. Procedures used to appoint a guardian and protectively place an individual must conform to the essentials of due process. Walworth County v. Therese B. 2003 WI App 223, 267 Wis. 2d 310, 671 N.W.2d 377, 03-0967. This section is not a hearsay exception and does not make inadmissible hearsay admissible but makes expert's opinion admissible even if the expert has relied on inadmissible hearsay in arriving at the opinion, as long as the hearsay is the type of facts or data reasonably relied on by experts in the particular field in forming opinions on the subject. A circuit court must be given latitude to determine when the underlying hearsay may be permitted to reach the trier of fact through examination of the expert with cautioning instructions for the trier of fact to head off misunderstanding and when it must be rigorously excluded altogether. Staskal v. Symons Corporation, 2005 WI App 216, 287 Wis. 2d 511, 706 N.W.2d 311, 04-0663. In an OWI prosecution, even if a defendant establishes a constitutional right to present an expert opinion that is based in part on portable breath test results, applying the St. George test, the right to do so is outweighed by the state's compelling interest to exclude that evidence. Permitting the use of that evidence as the basis for an expert opinion would render meaningless the legislature's forbidding of that evidence in OWI prosecutions under s. 343.303, an act that promotes efficient investigations of suspected drunk driving incidents and furthers the state's compelling interest in public safety on its roads. State v. Fischer, 2010 WI 6, 322 Wis. 2d 265, 778 N.W.2d 629, 07-1898. That part of this section that a properly qualified expert witness may rely on inadmissible material if that material is "of a type reasonably relied upon by experts in the particular field in forming opinions or inferences upon the subject," rests on the commonsense reality that a testifying expert could not be required to replicate all of the experiments and personally make all of the observations either underlying the development of the expert's field or otherwise relevant to the expert's opinion. Permitting the expert to rely on inadmissible material in accordance with this section does not violate a defendant's right to confrontation. State v. Heine, 2014 WI App 32, 354 Wis. 2d 1, 844 N.W.2d 409, 13-1022. An evaluation of drug testing procedures. Stein, Laessig, Indriksons, 1973 WLR 727.