Guide to Evidence  Section 703. Bases of opinion testimony by experts

The facts or data in the particular case upon which an expert witness bases an opinion or inference may be those perceived by or made known to the witness at or before the hearing. These include (a) facts observed by the witness or otherwise in the witness’s direct personal knowledge; (b) evidence already in the record or that will be presented during the course of the proceedings, which facts may be assumed to be true in questions put to the witness; and (c) facts or data not in evidence if the facts or data are independently admissible in evidence and are a permissible basis for an expert to consider in formulating an opinion. However, in criminal cases, a prosecution expert’s opinion may not depend on testimonial hearsay unless the declarant is unavailable and the defendant had a prior opportunity for cross-examination of the declarant.

This section is derived from Commonwealth v. Gordon, 496 Mass. 554, 575 (2025); Department of Youth Servs. v. A Juvenile, 398 Mass. 516, 531 (1986); LaClair v. Silberline Mfg. Co., 379 Mass. 21, 32 (1979); and Commonwealth v. Russ, 232 Mass. 58, 73 (1919). See Commonwealth v. Piantedosi, 478 Mass. 536, 541–546 (2017). Unlike Fed. R. Evid. 703, which permits opinions based on inadmissible evidence of a type reasonably relied upon by experts in the relevant field, Massachusetts law requires the facts or data underlying an opinion to be independently admissible, even if not actually admitted. Department of Youth Servs. v. A Juvenile, 398 Mass. at 530–531. However, in criminal cases, the opinion of an expert for the Commonwealth generally may not be “dependent upon” facts or data that are testimonial hearsay because such an opinion would violate the defendant’s confrontation rights. Gordon, 496 Mass. at 575 & n.25. The holding in Gordon does not extend to civil cases or to witnesses called by defendants in criminal cases.

An opinion may be based on a combination of the three sources of facts or data set forth in this section. See, e.g., Commonwealth v. Gallett, 481 Mass. 662, 682 (2019) (medical examiner may testify that type of injury he personally observed was consistent with Commonwealth’s theory, presented in a hypothetical question, based on facts in evidence, of manner in which injury was inflicted).

Opinion based on personal knowledge

An expert may base an opinion on facts within the expert’s personal knowledge. Commonwealth v. Barry, 481 Mass. 388, 407-408 (2019) (director of laboratory where DNA testing took place, who participated in analysis of samples and wrote report detailing his conclusions, properly offered expert testimony); Sacco v. Roupenian, 409 Mass. 25, 30 (1990) (physician’s opinion about progress of cancer based on observations during treatment); McLaughlin v. Bernstein, 356 Mass. 219, 222 (1969) (examination of machine). Compare Commonwealth v. Barbosa, 477 Mass. 658, 667-669 (2017) (expert properly testified, based on personal knowledge, that defendant belonged to gang), with Commonwealth v. Wardsworth, 482 Mass. 454, 466-470 (2019) (reversible error for expert to testify that defendant was gang member based solely on personal observations that defendant associated with suspected gang members).

Opinion based on evidence in the record

An expert may testify, in response to a hypothetical question or otherwise, to an opinion that is based on evidence already admitted. Commonwealth v. Burgess, 450 Mass. 422, 435–436 (2008). The trial judge may permit an expert to express an opinion de bene based on counsel’s representation that evidence of the supporting facts will be introduced later in the trial. Harris-Lewis v. Mudge, 60 Mass. App. Ct. 480, 485–486 (2004). If such evidence is not forthcoming, the court should strike the opinion and instruct the jury to disregard it. Id. If the opposing party fails to move to strike the opinion, it may be considered for whatever probative value the jury deems appropriate. Wilborg v. Denzell, 359 Mass. 279, 283 (1971); Commonwealth v. Salyer, 84 Mass. App. Ct. 346, 356 (2013).

When there is a factual dispute, a hypothetical question posed to an expert witness must identify which set of facts or portions of the record form the basis for the expert’s opinion. See Connor v. O’Donnell, 230 Mass. 39, 42 (1918). A hypothetical question that misstates the evidence is properly excluded. Commonwealth v. Rosario, 21 Mass. App. Ct. 286, 289 (1985).

Opinion based on independently admissible evidence

An expert witness may base an opinion on facts or data that would be admissible, even if not actually admitted at trial. The facts or data must be of the type relied upon by experts in the field. Department of Youth Servs. v. A Juvenile, 398 Mass. 516, 528–531 (1986). But see Commonwealth v. Gordon, 496 Mass. 554, 581–584 (2025) (in criminal cases, expert opinion dependent on testimonial hearsay violates defendant’s confrontation rights). In determining whether facts or data are independently admissible, the court must determine whether the underlying facts or data would potentially be admissible in any form through appropriate witnesses. Such witnesses need not be immediately available in court to testify. See Commonwealth v. Markvart, 437 Mass. 331, 337–338 (2002). But see Custody of Michel, 28 Mass. App. Ct. 260, 265–267 (1990) (permitting broader basis for testimony and reports of court-appointed investigators under G. L. c. 119, § 24). Thus, a psychologist called by the defense in a murder trial could opine on the defendant’s mental impairment at the time of the offense based on the witness’s interview with the defendant five weeks after the killings and the contents of police and medical records, but not on the basis of a psychiatrist’s earlier “preliminary diagnosis” that was not shown to be reliable and independently admissible. Commonwealth v. Waite, 422 Mass. 792, 803–804 (1996).

An expert who relies on independently admissible facts that are not in evidence is not ordinarily permitted to testify to the basis of the opinion on direct examination. Commonwealth v. Ronchi, 491 Mass. 284, 301 (2023). "The rationale for this limitation is to prevent the proponent of the expert testimony from 'import[ing] inadmissible hearsay into the trial.'" Commonwealth v. Chism, 495 Mass. 358, 378–379 (2025), quoting Commonwealth v. Goddard, 476 Mass. 443, 448 (2017). “[A]n expert witness may not, under the guise of stating the reasons for his opinion, testify to matters of hearsay in the course of his direct examination unless such matters are admissible under some statutory or other recognized exception to the hearsay rule.” Commonwealth v. Nardi, 452 Mass. 379, 392 (2008), quoting Grant v. Lewis/Boyle, Inc., 408 Mass. 269, 273 (1990). See Matter of J.P., 486 Mass. 117, 122 n.11 (2020) (on direct examination, expert may not testify about statements within medical record that formed basis of expert’s opinion where record itself was not admitted). But see Commonwealth v. Asenjo, 477 Mass. 599, 607–609 (2017) (error to exclude expert testimony regarding battered woman syndrome where G. L. c. 233, § 23F, provides independent statutory basis for admission of evidence; statute is more permissive than common law embodied in Section 703 and permits defendant to offer expert testimony if certain specified defenses are asserted).

The limitation on the direct-examination testimony of expert witnesses operates in both civil and criminal cases and applies to both sides. Vassallo v. Baxter Healthcare Corp., 428 Mass. 1, 15–16 (1998). See Commonwealth v. Chappell, 473 Mass. 191, 204 (2015) (this evidentiary rule does not violate defendant’s right to present a full defense). On cross-examination, the opposing party may choose to elicit the complete basis for an expert’s opinion, including whether any underlying facts or data were not from personal knowledge. Nardi, 452 Mass. at 390. Even if an expert is aware of certain facts not in evidence, a judge may exclude reference to those facts as unfairly prejudicial if they were not relied upon as part of the expert’s opinion, do not clarify or discredit the opinion, and serve only to focus the jury on those facts. Commonwealth v. Anestal, 463 Mass. 655, 667–668 (2012) (prior bad acts excluded). The decision to elicit admissible facts not in evidence that form the basis for an expert’s opinion belongs solely to the opposing party, and it is improper for the judge to request testimony about those facts. See Matter of P.R., 488 Mass. 136, 142–144 (2021) (reversible error to permit psychiatrist to testify on direct examination to hearsay basis for his opinion, even when basis was solicited at trial judge’s request).

Cross-Reference: Section 705, Disclosure of Facts or Data Underlying Expert Opinion.

Substitute experts and the confrontation clause

Hearsay statements of fact and opinion contained in forensic reports—such as drug certificates, autopsy reports, and DNA analyses—created to be used against the accused in investigating or prosecuting a crime are inadmissible against the defendant in a criminal case unless the author is present in court and subject to cross-examination or the author is unavailable and there was a prior opportunity for cross-examination. Melendez-Diaz v. Massachusetts, 557 U.S. 305, 309–311 (2009); Commonwealth v. Nascimento-Depina, 496 Mass. 1, 6-7 (2025) (substitute DNA expert testimony conveying findings and analyses of analyst who conducted testing inadmissible as testimonial hearsay); Commonwealth v. Nardi, 452 Mass. 379, 391–394 (2008). 

In Commonwealth v. Gordon, 496 Mass. 554, 575 (2025), the court, applying Smith v. Arizona, 602 U.S. 779, 795 (2024), held that a substitute expert’s opinion that depends on testimonial hearsay generated by a nontestifying analyst violates the confrontation clause, even when the testimonial hearsay is not elicited on direct examination. In Gordon, a forensic analyst who did not testify at trial had performed gas chromatography-mass spectrometry (GC-MS) testing on an unidentified substance. A State police crime laboratory supervisor, who performed a technical and administrative review of the testing analyst’s work but did not participate in or observe the testing, testified as a substitute expert in the criminal case to an opinion identifying the controlled substance. Gordon, 496 Mass. at 555–559.

The court first held that the notes made by the original analyst, recording the procedures she followed in testing the substance, were testimonial hearsay because the analyst reasonably would anticipate that her notes and findings would be available for use at trial. Id. at 565–570. The court next announced that the “independent” opinion of a substitute expert violates the confrontation clause “where the substitute expert’s opinion is dependent on the truth of a nontestifying analyst’s testimonial hearsay.” Id. at 574. Applying this rule, the court noted that the raw data GC-MS output itself was not hearsay because it was machine generated, id. at 577–578; however, because the substitute expert relied not only on the raw data but also on the truth of the original analyst’s statements regarding the procedures and protocols followed in generating the data, the substitute expert’s testimony was not truly independent and violated the confrontation clause, id. at 576, 580–581.

Finally, addressing the Massachusetts common-law evidentiary rule, see Department of Youth Servs. v. A Juvenile, 398 Mass. 516, 531–532 (1986), the court reexamined its line of cases holding that, in criminal cases, a substitute expert may offer an opinion based on the testimonial hearsay of another expert without violating the defendant’s confrontation rights so long as the other expert’s out-of-court statements are not introduced during direct examination of the substitute expert. See Commonwealth v. Greineder, 464 Mass. 580, 584–589, cert. denied, 571 U.S. 865 (2013) (collecting cases). The court held as follows:

“Following Smith, this aspect of our evidentiary rule, which permits a substitute expert who is a supervisor of the crime lab to provide an opinion regarding raw data generated by an absent analyst that depends on the truth of the testimonial hearsay of an absent analyst as to the processes and protocols she says she followed to obtain the data, no longer comports with the right of confrontation, and the ad-mission of such expert opinion testimony is an error of constitutional dimension.”

Gordon, 496 Mass. at 584.

The constitutional rule announced in Gordon is a new rule and does not apply to cases that became final before Smith v. Arizona, 602 U.S. 779, 795 (2024). Gordon, 496 Mass. at 587–588. Rather, it “applies prospectively and retroactively to those cases that were active or pending on direct appeal as of the date of the issuance of Smith.” Id.

Medical examiner

A substitute medical examiner may offer an opinion even though it is based in whole or in part on evidence collected or created by the absent medical examiner. See Commonwealth v. Seino, 479 Mass. 463, 466–468 (2018) (substitute medical examiner may offer opinion as to cause of death based upon review of independently admissible documents contained in original medical examiner’s file); Commonwealth v. Nardi, 452 Mass. 379, 388 (2008). The original autopsy report, notes, and photographs provide the defendant with a “meaningful opportunity” to cross-examine the substitute witness about possible flaws in the substitute’s own opinion. Commonwealth v. Tassone, 468 Mass. 391, 400 (2014). The Commonwealth is not required to show that the medical examiner who performed an autopsy is unavailable for a substitute medical examiner to testify. Commonwealth v. Leiva, 484 Mass. 766, 792 (2020).

Meaningful opportunity to cross-examine

The Massachusetts common law of evidence is more protective of confrontation rights than the Sixth Amendment to the United States Constitution in that it requires that the defendant have “a meaningful opportunity to cross-examine the expert about her opinion and the reliability of the facts or data that underlie her opinion.” Tassone, 468 Mass. at 399–402. Thus, in Tassone, the court held that where the substitute DNA analyst was not affiliated with the laboratory where the DNA testing was conducted and there was no showing that she had any personal knowledge of that lab’s evidence-handling protocols, the defendant was denied the opportunity to explore through cross-examination whether the testing was flawed. See Commonwealth v. Barry, 481 Mass. 388, 407–408 (2019) (director of laboratory that conducted DNA testing was not substitute expert, despite not being person who physically tested samples, because he participated in analysis of samples and testified about report detailing his conclusions). Compare Commonwealth v. Sanchez, 476 Mass. 725, 734 (2017) (fire inspector who was present for electrician’s inspection of arson site could testify and be meaningfully cross-examined about his own observations), with Commonwealth v. Jones, 472 Mass. 707, 715–716 (2015) (where DNA expert’s knowledge of how DNA samples had been collected was derived from form completed by person who had collected the specimens from victim’s body, no meaningful opportunity to cross-examine witness). Under Gordon, 496 Mass. at 582–584, however, even an expert from the same laboratory where the testing was conducted cannot offer an opinion that depends on testimonial hearsay from the person who conducted the test.

DNA analyst

Where the prosecution offers an opinion about a DNA profile match, “a meaningful opportunity for cross-examination means that a defendant must have the opportunity substantively to explore the ‘risk of evidence being mishandled or mislabeled, or of data being fabricated or manipulated, and . . . whether the expert’s opinion is vulnerable to these risks.’” Tassone, 468 Mass. at 400, quoting Commonwealth v. Barbosa, 457 Mass. 773, 791 (2010), cert. denied, 563 U.S. 990 (2011). The testifying expert may testify from personal knowledge about the testing laboratory’s procedures but may not give an opinion about a DNA match if the opinion depends on testimonial hearsay generated by an absent DNA analyst. See Gordon, 496 Mass. at 584. An expert who has no knowledge of how the sample was collected cannot testify to the location from which the sample was collected. Jones, 472 Mass. at 716–717 n.3 (no meaningful opportunity to cross-examine testifying DNA expert about how specimen was collected).

Cross-Reference: Section 702, Testimony by Expert Witnesses; Section 705, Disclosure of Facts or Data Underlying Expert Opinion; Introductory Note to Article VIII, Hearsay.