Abrams, J.
The juvenile is in a secure facility (Butler Center) run by the Department of Youth Services (department). Although he is over eighteen, he is in the department’s custody because a jury of six determined that if discharged from the department he “would be physically dangerous to the public because of his mental disorder or abnormality.” See G. L. c. 120, §§ 16-20 (1984 ed.). The juvenile appeals, alleging that the statutory provisions under which he is held are unconstitutional; that the judge erred in permitting a psychiatrist to testify because he (the juvenile) was not warned that his statements to the psychiatrist could be used to extend his commitment to the department; that it was error to admit the doctor’s opinion
because his opinion was based on hearsay reports and conversations and not facts and data admitted in evidence; and that the judge erred in two other rulings. We granted direct appellate review on our own motion. For the reasons stated herein, we conclude there must be a new trial. We therefore reverse and remand for a new trial.
We summarize the prior proceedings. At age seventeen, the juvenile was adjudicated delinquent on a complaint alleging indecent assault and battery on a child under fourteen. See G. L. c. 265, § 13B. He was committed to the department, and shortly thereafter the department requested permission from the District Court to extend the juvenile’s commitment beyond his eighteenth birthday. See G. L. c. 120, §§ 16 & 17.
After a hearing pursuant to G. L. c. 120, § 18,
a District Court
judge concluded that the juvenile “would be physically dangerous to the public because of his mental disorder or abnormality if he were discharged from the control of the [department] at this time.” The judge extended the juvenile’s commitment to the department beyond his eighteenth birthday. Pursuant to G. L. c. 120, § 20,
the juvenile appealed to a six-person jury
which also concluded that the juvenile would be “physically dangerous to the public” if released from the department.
We summarize the facts. Doctor Lawrence Gibson, a psychiatrist, said that he evaluated the juvenile on behalf of the department and the Butler Center (center), a strict security treatment center in which the juvenile was residing as a result of the adjudication of delinquency.
Doctor Gibson’s evaluation consisted of several meetings with the juvenile, meetings with the juvenile’s therapist, meetings with the criminal coordinator at the center, and of a review of the juvenile’s records at the center. Based on his evaluation, Dr. Gibson concluded that the juvenile had “disorders of emotion and perception” which manifested themselves by an “inability to cope with the demands of life,” inappropriate anger, stealing, lying, self-doubt, and aggression toward others. The doctor stated that in his opinion “there’s a great likelihood that those type of behaviors [i.e., ‘inappropriate contact of a sexual nature in young children’] will be repeated again [unless the juvenile undergoes] a substantial change in his personality structure.” He opined that the juvenile presented a “very significant danger [to] the physical well being of the community.”
Doctor Richard Lazur, a psychologist who worked as a clinical coordinator at the center, was of the opinion that the
juvenile suffered from a “severe emotional disturbance” which required in-patient treatment. Doctor Lazur characterized the juvenile as mild-mannered, compliant and “very nice” and noted that he “tends to not cause behavioral disturbances” or express anger. Doctor Lazur stated that the juvenile is impulsive, feels victimized, feels that he “has no responsibility for what goes on with his life,” and has “difficulty getting along with people and forming adequate social relationships with them.” He further opined that the juvenile’s feelings of victimization, his mild-mannered personality and his denial of responsibility for his actions are characteristic of child molesters. Doctor Lazur concluded that the juvenile presented “a very severe threat to the public, especially to the children in the community.”
The charge for indecent assault and battery on a child under fourteen was the only charge ever filed against the juvenile. Reports from the juvenile’s teachers at the center indicated that his classroom performance and behavior were favorable. A current and a former member of the staff at the Boys’ Shelter in Brockton, where the juvenile resided prior to his placement at the center, said that, while the juvenile was at the Boys’ Shelter, he had no restrictions imposed upon him for infraction of rules, did not attempt to escape, and did not assault or threaten to assault anyone.
1.
Constitutionality of G. L. c. 120, §§ 16-20.
The juvenile challenges the constitutionality of G. L. c. 120, §§ 16-20, arguing that (1) it is unconstitutionally vague because it does not adequately define or establish guidelines for determining that a defendant is “dangerous to the public” and suffers from a “mental or physical deficiency, disorder or abnormality”; and (2) it unconstitutionally permits cruel and unusual punishment by providing for criminal commitment of a person for an unlimited period because of his mental or physical status. Although the juvenile did not properly challenge the constitutionality of G. L. c. 120, §§ 16-20, prior to trial, he did raise the constitutionality of the statute in his motion for directed verdict in this posture. We first consider the statute on an as-applied basis. See
Commonwealth
v.
Jasmin,
396 Mass. 653, 655 (1986).
The juvenile challenges the constitutionality of the statute on vagueness grounds. “A law is void for vagueness if persons ‘of common intelligence must necessarily guess at its meaning and differ as to its application.’
Smith
v.
Goguen,
415 U.S. 566, 572 n.8 (1974), quoting
Connally
v.
General Constr. Co.,
269 U.S. 385, 391 (1926). Vague laws violate due process because individuals do not receive fair notice of the conduct proscribed by a statute,
Papachristou
v.
Jacksonville,
405 U.S. 156, 162 (1972), and because vague laws that do not limit the exercise of discretion by officials engender the possibility of arbitrary and discriminatory enforcement,
Grayned,
v.
Rockford,
408 U.S. 104, 108-109 & n.4 (1972).”
Commonwealth
v.
Jaffee, ante
50, 54 (1986), quoting
Caswell v. Licensing Comm’n for Brockton,
387 Mass. 864, 873 (1983). Further, “[i]t is well established that vagueness challenges to statutes which do not involve First Amendment freedoms must be examined in the light of the facts of the case at hand.”
United States
v.
Powell,
423 U.S. 87, 92 (1975), quoting
United States
v.
Mazurie,
419 U.S. 544, 550 (1975). See
Commonwealth
v.
Adams,
389 Mass. 265, 271 (1983).
The facts of this case include evidence before the judge, and then the jury of six, that due to the juvenile’s mental disorder or disturbance, he is “physically dangerous to the public.” This evidence was presented in the form of two expert opinions. The experts opined that the juvenile suffered from a “severe emotional disturbance,” and these opinions were admitted without objection from the juvenile.
As this court recognized, “a State may validly exercise its police power to confine persons who are adjudicated mentally ill and dangerous.”
Thompson
v.
Commonwealth,
386 Mass. 811, 815 (1982).
See
Addington
v.
Texas,
441 U.S. 418, 429 (1979).
Cf.
O’Connor
v.
Donaldson,
442 U.S. 563, 576 (1975) (a State may not hold nondangerous mentally ill persons against their will). Therefore, in light of the evidence presented at trial, a vagueness challenge to the statute fails.
We next turn to the juvenile’s claim that G. L. c. 120, §§ 16-20, is unconstitutional because it subjects a person to cruel and unusual punishment for his mental or physical status as one who is “physically dangerous to the public because of [his] mental or physical deficiency, disorder or abnormality.”
Robinson
v.
California,
370 U.S. 660 (1962).
Unlike the statute in
Robinson,
however, which punished addiction to narcotics, this statute confines an individual due to a mental condition that renders him dangerous to the public. A State may validly exercise its police power to confine persons who are adjudicated mentally ill and dangerous.
Thompson
v.
Commonwealth, supra.
While we have had few opportunities to construe c. 120, we are guided by the more explicit provisions of G. L. c. 123, a statute which regulates commitment of a mentally ill person. General Laws c. 120, §§ 16-20, differs from c. 123 in that it is concerned with the treatment of juveniles. See
Department of Youth Servs.
v.
A Juvenile,
384 Mass. 784, 791-793 (1983). Both G. L. c. 123 and c. 120, §§ 16-20, however, are statutes which have as their purpose the protection of others from physical harm and the treatment of the mentally ill person. See
Department of Youth Servs., supra
at 791;
Commonwealth
v.
Nassar,
380 Mass. 908, 918 (1980). Therefore, in construing G. L. c. 120, we refer to c. 123’s definitions of similar words and phrases because the two statutes are concerned with the same general subject matter. See
Commonwealth
v.
Baker,
368 Mass. 58, 69 (1975);
Commonwealth
v.
Flynn,
285 Mass. 136, 140 (1934).
Under G. L. c. 123, § 8, an individual may be confined if the fact finder concludes that “such person is mentally ill, and ... the discharge of such person . . . would create a likelihood of serious harm.” These standards are applicable to proceedings under c. 120. General Laws c. 120, § 17, requires a finding that a person is “dangerous to the public” as a result of a “mental or physical deficiency, disorder or abnormality.” In construing these statutes, the term “mental illness” promulgated under c. 123
shall be used to define c. 120’s requirement of “mental or physical deficiency, disorder or abnormality.” Similarly, the c. 123 definition of “likelihood of serious harm”
is the appropriate definition to be employed in giving meaning to “dangerous to the public” as used in c. 120.
2.
Lamb warnings.
The juvenile claims that it was error to permit Dr. Gibson, the psychiatrist, to testify because the doctor did not warn him prior to interviewing him that their conver
sations were not confidential and could be used against him in a proceeding to extend his commitment.
Commonwealth
v.
Lamb,
365 Mass. 265 (1974).
Commonwealth
v.
Barboza,
387 Mass. 105, cert. denied, 459 U.S. 1020 (1982). We agree that the juvenile should have been warned. Because the juvenile did not raise this argument below, we next determine whether the failure to deliver such warnings created a substantial likelihood of a miscarriage of justice.
Commonwealth
v.
Festa,
388 Mass. 513, 515 (1983). We conclude that it does.
In
Commonwealth
v.
Lamb, supra
at 270, we said that, in a proceeding under G. L. c. 123A for commitment of a person convicted of a violent sex crime or a sexually dangerous person, G. L. c. 233, § 20B,
established a patient’s right to “keep privileged any communications made to a court-appointed
psychotherapist in the case of a court-ordered examination, absent a showing that he was informed that the communication would not be privileged and thus, inferentially, that it would be used at the commitment hearing.” Because the purpose of G. L. c. 120, §§ 16-20, is similar to that of G. L. c. 123A — to protect the public “against future anti-social behavior by the offender” and to rehabilitate him
[Commonwealth
v.
Rodriguez,
376 Mass. 632, 646 (1978)] — there is no reason not to apply the same procedural safeguards to proceedings regarding sexually dangerous persons to those involving extended commitments for persons adjudicated delinquent.
The Commonwealth makes the anomalous argument that, because it did not seek a court order for the psychiatric evaluation, the
Lamb
warnings should not be required. That argument is without merit. The warnings are required by reason of G. L. c. 233, § 20B, see
Commonwealth
v.
Lamb, supra
at 267-268, and do not depend on whether the Commonwealth chooses to interview a person on its own initiative or decides to seek court permission. The requirement of warnings is not a matter of prosecutorial discretion.
The Commonwealth also contends that
Lamb
warnings were not required because Dr. Gibson did not disclose any communications from the juvenile at trial. Doctor Gibson, however, gave a diagnosis, which was based, at least in part, on conversations with the juvenile which were not preceded by any warnings. The juvenile was not warned that his interviews with Dr. Gibson were not confidential and could be used against him in a proceeding to extend his commitment. As we read the
Lamb
case, those interviews, if they were to be the basis of the expert’s opinion, should have been preceded by the
Lamb
warnings.
“We cannot conclude in all fairness that the jury would not possibly have found this evidence significant in their consideration of the issue of [the juvenile’s mental disorder and whether he was physically dangerous to the public].”
Commonwealth
v.
Schulze,
389 Mass. 735, 742 (1983).
Because there is a substantial risk of a miscarriage of justice, we reverse and remand for a new trial.
3. We turn to issues likely to recur at a new trial.
a.
The expert opinion.
During the trial the Commonwealth’s expert said that he reviewed the records at the Butler Center which included “family history, psychology . . . testing, medical and neurological histories as well as interviews . . . conducted by the [department’s] case workers with [the juvenile’s] mother.” The expert relied on these reports as well as his own interviews with the juvenile. None of the reports was offered in evidence. On appeal, the juvenile asserts that the judge should not have permitted the expert to state his opinion because the expert relied on facts not in evidence.
The settled and traditional rule in Massachusetts is that “[t]he competency of an expert witness to testify to his opinion rests upon unusual knowledge and extraordinary experience, superior to that of ordinary persons. The witness, being qualified in this particular, then may base his opinion upon facts observed by himself or within his own knowledge and testified to by himself or upon facts assumed in the questions put to him and supported either by admitted facts or by the testimony of other witnesses already given or to be given at the trial, or upon facts derived partly from one source and partly from the other.”
Commonwealth
v.
Russ,
232 Mass. 58, 73 (1919). See
LaClair
v.
Silberline Mfg. Co.,
379 Mass. 21, 32 (1979);
Commonwealth
v.
Harrison,
342 Mass. 279, 287-288 (1961); McCormick, Evidence § 14 (3d ed. 1984); P.J. Liacos, Massachusetts Evidence 111 (5th ed. 1981). The Commonwealth cites to Proposed Mass. R. Evid. 703 as enlarging the basis for expert opinion. Because there is to be a new trial, we turn to consideration of Proposed Mass. R. Evid. 703. See Announcement Concerning the Proposed Massachusetts Rules of Evidence (December 30, 1982).
Proposed Mass. R. Evid. 703 provides: “BASES OF OPINION TESTIMONY BY EXPERTS 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 him 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.” The proposed rule was taken verbatim from Fed. R. Evid. 703.
The first sentence of Proposed rule 703 tracks our current practice. The second sentence, however, which provides that an expert may base an opinion on facts or data
not admissible
in
evidence
“[i]f [the evidence is] of a type reasonably relied on by experts in the particular field” radically departs from our current practice. The Federal Advisory Committee states: “[T]he rule is designed to broaden the basis for expert opinions beyond that current in many jurisdictions and to bring the judicial practice into line with the practice of the experts themselves when not in court. Thus a physician in his own practice bases his diagnosis on information from numerous sources and of considerable variety, including statements by patients and relatives, reports and opinions from nurses, technicians and other doctors, hospital records, and X rays. Most of them are admissible in evidence, but only with the expenditure of substantial time in producing and examining various authenticating witnesses. The physician makes life-and-death decisions in reliance upon them. His validation, expertly performed and subject to cross-examination, ought to suffice for judicial purposes.” See Advisory Committee’s Note to Fed. R. Evid. 703, 28 U.S.C. App. 711 (1982 ed.).
“The proper inquiry is not what the court deems reliable, but what experts in the relevant discipline deem it to be.”
In re Japanese Elec. Prods.,
723 F.2d 238, 277 (3d Cir. 1983), cert. granted on other grounds, 471 U.S. 931 (1985), reversed and remanded sub nom.,
Matsushita Elec. Indus. Co.
v.
Zenith Radio Corp.,
475 U.S. 574 (1986). See
Indian Coffee Corp.
v.
Procter & Gamble Co.,
752 F.2d 891, 895-897 (3d Cir. 1985);
Wilder Enterprises
v.
Allied Artists Pictures Corp.,
632 F.2d 1135, 1143-1144 (4th Cir. 1980). The rule has been construed broadly by the Federal courts., See, e.g.,
United States
v.
Baca,
687 F.2d 1356, 1361 (10th Cir. 1982) (expert could give opinion as to defendant’s competence based on another doctor’s evaluations);
United States
v.
Lawson,
653
F.2d 299, 301-302 (7th Cir. 1981) (psychiatrist could base his opinion on staff reports, defendant’s interviews with other physicians, information received from United States Marine Corps, reports from the F.B.I. and “ ‘a large amount of information’ furnished by the United States Attorney’s Office”);
Mannino
v.
International Mfg. Co.,
650 F.2d 846, 853 (6th Cir. 1981) (expert in biomechanical engineering could testify on improper design based on literature and information furnished by plaintiff’s attorney);
United States
v.
Bilson,
648 F.2d 1238, 1239 (9th Cir. 1981) (psychiatrist could base his opinion of defendant’s sanity on psychological tests administered by unlicensed psychologist);
Bauman
v.
Centex Corp.,
611 F.2d 1115, 1120 (5th Cir. 1980) (expert could rely in part on research done in university library);
O’Gee
v.
Dobbs Houses, Inc.,
570 F.2d 1084, 1089 (2d Cir. 1978) (physician as expert allowed to testify to patient’s version of other doctors’ opinions, court noted that expert also had reports by two of the other doctors as well as a hospital report).
American Universal Ins. Co.
v.
Falzone,
644 F.2d 66, 66 (1st Cir. 1981) (no error to permit expert to consider “contemporaneous and on-the-scene opinions of other investigators on his team . . . as to [their] portion of the investigation”). “The way to combat such evidence is by cross-examination, not claiming foul.”
Knightsbridge Mktg. Servs., Inc.
v.
Promociones Y Proyectos,
728 F.2d 572, 576 (1st Cir. 1984). The various opinions construing rule 703 also may reflect the fact that in the Federal courts some of the evidence would be admissible under the
residual exceptions for hearsay. See Fed. R. Evid. 803(24)
and 804(b)(5).
Federal Rule of Evidence 703 has been described as raising a “serious potential for abuse.” 1983 A.B.A. Sect. Litig., Emerging Problems Under the Federal Rules of Evidence 204, 210. “[B]y permitting experts to base opinions on data not admissible in evidence, Rule 703 encourages litigators to look for experts who will base their opinions, at least in part, on evidence that is otherwise inadmissible, but which the proponents of the experts would like the juries to hear. Whether or not using Rule 703 in this manner is an abuse, it is an increasingly common tactic.” Arnolds, Federal Rule of Evidence 703; The Back Door Is Wide Open, 20 F. 1, 18 (1984). “Rule 703 allows the expert to give a courtroom opinion based on facts
or data which may be, although need not be, admissible evidence. The broad language of rule 703 has, however, created uncertainty concerning the admissibility of material relied upon by the expert. The uncertainty focuses on whether the evidence is received as substantive proof, as a new exception to the hearsay rule, or whether the material may be mentioned only for the limited purpose of demonstrating what data the expert relied upon. This controversy has continued to plague the courts.” (Footnotes omitted.) Carlson, Collision Course In Expert Testimony: Limitations On Affirmative Introduction of Underlying Data, 36 U. Fla. L. Rev. 234, 243 (1984). “Rule 703 is an undesignated thirtieth exception to the prohibition against hearsay. It authorizes trial judges to permit an expert witness to base his (or her) opinion testimony on inadmissible data conveyed to the witness outside the courtroom. And some of the cases go a long step farther and allow the expert, on his direct examination, to describe this inadmissible data to the factfinder. That is what is known as a back door and no matter how many limiting instructions you may have, it is an ominously large door.” Waltz, Evidence is Dead, Wigmore Obsolescent: Long Live Judicial Discretion.
The rule “permits the jury to hear information that would normally be considered improper evidence.” Rossi, Modem Evidence and the Expert Witness, 12 Litigation No. 1, 18, 24 (1985).
Because of the problems now arising under mle 703, we are not persuaded we should accept the principles of the proposed rule. We believe, however, that we should take a modest step by permitting an expert to base an opinion on facts or data not in evidence if the facts or data are independently admissible and are a permissible basis for an expert to consider in formulating an opinion. Such a change will eliminate the necessity of producing exhibits and witnesses whose sole function is to construct a proper foundation for the expert’s opinion.
If a party believes that an expert is basing an opinion on inadmissible facts or data, the party may request a voir dire to determine the basis of the expert opinion. If the facts or data are admissible and of the sort that experts in that specialty reasonably rely on in forming their opinions, then the expert may state that opinion without the facts or data being admitted in evidence.
Because the facts or data on which an expert may rely in formulating an opinion may not be in evidence, we next consider Proposed Rule 705. Proposed Mass. R. Evid. 705 provides: “The expert may testify in terms of opinion or inference and give his reasons therefor without prior disclosure of the underlying facts or data, unless the court requires otherwise. The expert may in any event be required to disclose the underlying facts or data on cross-examination.” The Advisory Committee’s Note on Proposed Rule 705 states that “[t]his is the Federal rule and is consistent with Massachusetts practice as regards the lack of necessity of prior disclosure of the basis for the opinion of the percipient expert.
Commonwealth
v.
Vaughn,
329 Mass. 333, 335 (1952). Its effect upon foundation for the non-percipient expert may not be substantial, because under Massachusetts practice the ‘scope and fullness of hypothetical questions is within the sound discretion of the trial judge.’ W. B. Leach & P. J. Liacos, Massachusetts Evidence 120 (4th ed. 1967). The rule is aimed principally at the abuse of the hypothetical question. It does not eliminate the availability of the hypothetical question, but only the requirement of its use. . . . The thrust of the rule is to leave inquiry regarding the basis of expert testimony to cross-examination, which is considered an adequate safeguard. Expanded discovery techniques are viewed as affording advance knowledge for purposes of cross-examination. See Mass. R. Civ. P. 26-37, as amended; Mass. R. Crim. P. 14 & 23, 378 Mass. 874, 893 (1979);
Commonwealth
v.
Lewinski,
367 Mass. 889, 901-903 (1975). Furthermore, it should be noted that it is within the court’s discretion to require an initial disclosure of underlying facts or data.”
We add a note of caution. The use of the word “may” in the second sentence does not permit a judge to exclude questions designed to elicit the underpinnings of the expert’s opinion.
b.
Motion to quash summons.
At trial, the juvenile applied to the court to issue a summons for Tony Medeiros, another juvenile at the center, on the ground that Medeiros’ testimony “will tend to show that [the juvenile] is not a person who poses a danger to the community.” The department moved to quash the summons on the ground that compelling Medeiros to testify “would be injurious to his health and further that the subject is suicidal.” After a hearing, the judge allowed the juvenile’s motion to suppress evidence of the incident (a fight at the Center), and allowed the department’s motion to quash summons of Mederios. On the record before us, we cannot say that the judge erred in excluding evidence of the incident.
c.
Motion for appointment of psychiatrist.
Pursuant to Mass. R. Crim. P. 41, 378 Mass. 918 (1979), the juvenile asked for the appointment of a psychiatrist to help him prepare his defense. He now contends that the trial judge erred in denying his motion. However, a motion for an expert was allowed “fee not to exceed $350.00.” The essence of the juvenile’s argument appears to be that “no psychiatrist would consider doing a psychiatric evaluation of [the juvenile] and then testify at trial for only $350.00.” Because the juvenile neither objected to the amount of the fee at trial nor presented any evidence that the sum was inadequate, there is no basis for concluding that there was an error in establishing the ceiling.
We reaffirm that an indigent defendant is entitled to extra costs and fees to prevent him from being disadvantaged in preparing or presenting his case adequately. See
Commonwealth
v.
Lockley,
381 Mass. 156, 160-161 (1980). The record does not disclose any reason to conclude that the amount awarded was insufficient to hire an expert. Nor is there any record support for the suggestion that more funds were needed to secure the testimony of an expert at trial. Thus, the juvenile’s argument on this issue is without merit. We reaffirm our position that “a ceiling may be established on the amount which may be expended.” Reporters’ Notes to Mass. R. Crim. P. 41, Mass. Ann. Laws, Rules of Criminal Procedure at 589 (1979).
The judgment extending the commitment order is vacated, and the case remanded to the jury of six for a new trial.
So ordered.