Meade, J.
National Grid, formerly KeySpan New England, LLC (self-insurer), appeals from a decision of the reviewing board of the Department of Industrial Accidents (board) affirming the award of temporary disability and medical benefits to William Goodwin (employee). At issue in this appeal is the application of the “major cause” standard of G. L. c. 152, § 1(7A), when an industrial injury combines with a noncompensable [643]*643preexisting condition to cause the employee’s disability.1 On appeal, the self-insurer claims that (1) the board’s decision, which failed to treat the impartial medical examiner’s opinion on causation as binding in the absence of any countervailing medical evidence, was arbitrary and capricious, and (2) the board’s interpretation of the statutory phrase “a major . . . cause” was contrary to law. We affirm.
1. Background. The employee was fifty-five years old at the time of the 2010 hearing before the administrative judge. He had worked his entire career for the self-insurer and its predecessors. During the prior seventeen years, he had worked as a pipefitter, which involved heavy physical labor. On December 29, 2008, while working overhead with two wrenches to repair a gas leak, the employee felt a “pop” in the back of his neck. Although he had experienced neck pain before, this pain was different, radiating down his right arm and much worse than the pain he had experienced in the past; he also experienced numbness in the arm. An MRI (magnetic resonance imaging) examination of his cervical spine revealed extensive degenerative disc disease and disc degeneration at multiple levels, with osteophyte formation, facet changes, and impingement of the thecal sac at the C4, C5, and C6 vertebrae, primarily on the right side. Surgery eventually provided relief, and the employee returned to work on a full duty basis on February 1, 2010.
An impartial medical exam was performed by Dr. Nabil Basta pursuant to G. L. c. 152, § 11 A. He diagnosed a cervical strain and aggravation of preexisting cervical spondylosis. Dr. Basta opined that the work injury was responsible for forty percent of the employee’s disability and sixty percent was attributable to the employee’s preexisting degenerative condition. His written report concluded that the work injury was not a “major cause” of the employee’s disability or surgery, but only a “minor [644]*644component.” Dr. Basta’s conclusion, as he later explained at his deposition, was based on his belief that to be a “major cause,” the work injury must account for more than fifty percent of the disability. The administrative judge, who had no other medical expert testimony before him, adopted the medical findings in Dr. Basta’s report, but concluded that the doctor used an improper legal standard in reaching his conclusion that the injury was not a “major cause.” The administrative judge found and ruled, based on all the evidence before him, that forty percent “is sufficient to be a major cause of the employee’s disability and need for treatment.” The self-insurer appealed to the reviewing board, which affirmed in a written decision that is now before us.
2. Standard of review. The Supreme Judicial Court has summarized the guiding principles as follows:
“An aggrieved party may seek judicial review of a decision of the board concerning workers’ compensation benefits. G. L. c. 152, § 12(2). As specified in. . . § 12(2), we review the board’s decision in accordance with the standards set forth in G. L. c. 30A, § 14(7)(a)-(d), (/), and (g). Scheffler’s Case, 419 Mass. 251, 257-258 (1994). Pursuant to these statutory standards, we may reverse or modify the board’s decision when it is ‘[i]n violation of constitutional provisions,’ ‘[i]n excess of the statutory authority or jurisdiction of the agency,’ ‘[bjased upon an error of law,’ ‘[mjade upon unlawful procedure,’ ‘[Unwarranted by facts found . . . where the court is constitutionally required to make independent findings of fact,’ or is ‘[arbitrary or capricious, an abuse of discretion, or otherwise not in accordance with law.’ G. L. c. 30A, § 14(7)(a)-(<7), (/), and (g). Under these standards, in determining whether the board properly affirmed the administrative judge’s findings, we consider ‘whether the decision is factually warranted and not “[arbitrary or capricious,” in the sense of having adequate evidentiary and factual support and disclosing reasoned decision making.’ Scheffler’s Case, supra at 258. . . . Where a statute is involved, ‘[ajlthough “[t]he interpretation of a statute by the agency charged with primary responsibility for administering it is entitled to substantial deference,”
[645]*645Gateley’s Case, 415 Mass. 397, 399 (1993), ultimately the “duty of statutory interpretation is for the courts.” Slater’s Case, 55 Mass. App. Ct. 326, 330 (2002).’ Moss’s Case, 451 Mass. 704, 709 (2008).”
Carpenter’s Case, 456 Mass. 436, 439 (2010). In addition, we “give due weight to the experience, technical competence, and specialized knowledge of the agency, as well as to the discretionary authority conferred upon it.” G. L. c. 30A, § 14, as amended by St. 1973, c. 1114, § 3. See Murphy’s Case, 53 Mass. App. Ct. 424, 427 (2001).
3. Discussion. The self-insurer claims that the board’s decision, which failed to defer to Dr. Basta’s opinion on causation, was arbitrary and capricious. We disagree. “Findings of fact, assessments of credibility, and determinations of the weight to be given the evidence are the exclusive function of the administrative judge.” Pilon’s Case, 69 Mass. App. Ct. 167, 169 (2007). Dr. Basta opined that because the work injury was responsible for only forty percent of the employee’s disability while sixty percent was attributable to the employee’s preexisting degenerative condition that was not job-related, the work injury was not a “major cause” under G. L. c. 152, § 1(7A). The administrative judge was under no obligation to adopt all of Dr. Basta’s opinion, especially because (as we discuss below) his conclusion was based on a legal misunderstanding of what qualifies as a “major cause.” See Clarici’s Case, 340 Mass. 495, 497 (1960) (single member and reviewing board free to adopt any portions of medical testimony they deem credible). In other words, as the board stated, it was not within the purview of the impartial medical examiner to define the legal scope of G. L. c. 152, § 1(7A). As the board determined, in light of Dr. Basta’s misunderstanding of the meaning of “major cause,” the administrative judge, who applied the correct standard to Dr. Basta’s assessment of the work injury as a forty percent cause, was warranted in concluding that the employee had met his burden of proof under § 1(7A). Indeed, where there are two identified causes of an employee’s disability, a forty percent cause may be a “major cause.” As such, the board’s decision was not arbitrary and capricious.
The self-insurer also claims that the board’s interpretation of [646]*646the phrase “major cause” was contrary to law. More specifically, the self-insurer claims that the board’s conclusion that a “major cause” can be less of a factor than the most important cause was erroneous and inconsistent with the plain meaning of the statute. We disagree.
In interpreting a statute, we look first to its language as “the principal source of insight into legislative intent.” Water Dept. of Fairhaven v. Department of Envtl. Protection, 455 Mass.
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Meade, J.
National Grid, formerly KeySpan New England, LLC (self-insurer), appeals from a decision of the reviewing board of the Department of Industrial Accidents (board) affirming the award of temporary disability and medical benefits to William Goodwin (employee). At issue in this appeal is the application of the “major cause” standard of G. L. c. 152, § 1(7A), when an industrial injury combines with a noncompensable [643]*643preexisting condition to cause the employee’s disability.1 On appeal, the self-insurer claims that (1) the board’s decision, which failed to treat the impartial medical examiner’s opinion on causation as binding in the absence of any countervailing medical evidence, was arbitrary and capricious, and (2) the board’s interpretation of the statutory phrase “a major . . . cause” was contrary to law. We affirm.
1. Background. The employee was fifty-five years old at the time of the 2010 hearing before the administrative judge. He had worked his entire career for the self-insurer and its predecessors. During the prior seventeen years, he had worked as a pipefitter, which involved heavy physical labor. On December 29, 2008, while working overhead with two wrenches to repair a gas leak, the employee felt a “pop” in the back of his neck. Although he had experienced neck pain before, this pain was different, radiating down his right arm and much worse than the pain he had experienced in the past; he also experienced numbness in the arm. An MRI (magnetic resonance imaging) examination of his cervical spine revealed extensive degenerative disc disease and disc degeneration at multiple levels, with osteophyte formation, facet changes, and impingement of the thecal sac at the C4, C5, and C6 vertebrae, primarily on the right side. Surgery eventually provided relief, and the employee returned to work on a full duty basis on February 1, 2010.
An impartial medical exam was performed by Dr. Nabil Basta pursuant to G. L. c. 152, § 11 A. He diagnosed a cervical strain and aggravation of preexisting cervical spondylosis. Dr. Basta opined that the work injury was responsible for forty percent of the employee’s disability and sixty percent was attributable to the employee’s preexisting degenerative condition. His written report concluded that the work injury was not a “major cause” of the employee’s disability or surgery, but only a “minor [644]*644component.” Dr. Basta’s conclusion, as he later explained at his deposition, was based on his belief that to be a “major cause,” the work injury must account for more than fifty percent of the disability. The administrative judge, who had no other medical expert testimony before him, adopted the medical findings in Dr. Basta’s report, but concluded that the doctor used an improper legal standard in reaching his conclusion that the injury was not a “major cause.” The administrative judge found and ruled, based on all the evidence before him, that forty percent “is sufficient to be a major cause of the employee’s disability and need for treatment.” The self-insurer appealed to the reviewing board, which affirmed in a written decision that is now before us.
2. Standard of review. The Supreme Judicial Court has summarized the guiding principles as follows:
“An aggrieved party may seek judicial review of a decision of the board concerning workers’ compensation benefits. G. L. c. 152, § 12(2). As specified in. . . § 12(2), we review the board’s decision in accordance with the standards set forth in G. L. c. 30A, § 14(7)(a)-(d), (/), and (g). Scheffler’s Case, 419 Mass. 251, 257-258 (1994). Pursuant to these statutory standards, we may reverse or modify the board’s decision when it is ‘[i]n violation of constitutional provisions,’ ‘[i]n excess of the statutory authority or jurisdiction of the agency,’ ‘[bjased upon an error of law,’ ‘[mjade upon unlawful procedure,’ ‘[Unwarranted by facts found . . . where the court is constitutionally required to make independent findings of fact,’ or is ‘[arbitrary or capricious, an abuse of discretion, or otherwise not in accordance with law.’ G. L. c. 30A, § 14(7)(a)-(<7), (/), and (g). Under these standards, in determining whether the board properly affirmed the administrative judge’s findings, we consider ‘whether the decision is factually warranted and not “[arbitrary or capricious,” in the sense of having adequate evidentiary and factual support and disclosing reasoned decision making.’ Scheffler’s Case, supra at 258. . . . Where a statute is involved, ‘[ajlthough “[t]he interpretation of a statute by the agency charged with primary responsibility for administering it is entitled to substantial deference,”
[645]*645Gateley’s Case, 415 Mass. 397, 399 (1993), ultimately the “duty of statutory interpretation is for the courts.” Slater’s Case, 55 Mass. App. Ct. 326, 330 (2002).’ Moss’s Case, 451 Mass. 704, 709 (2008).”
Carpenter’s Case, 456 Mass. 436, 439 (2010). In addition, we “give due weight to the experience, technical competence, and specialized knowledge of the agency, as well as to the discretionary authority conferred upon it.” G. L. c. 30A, § 14, as amended by St. 1973, c. 1114, § 3. See Murphy’s Case, 53 Mass. App. Ct. 424, 427 (2001).
3. Discussion. The self-insurer claims that the board’s decision, which failed to defer to Dr. Basta’s opinion on causation, was arbitrary and capricious. We disagree. “Findings of fact, assessments of credibility, and determinations of the weight to be given the evidence are the exclusive function of the administrative judge.” Pilon’s Case, 69 Mass. App. Ct. 167, 169 (2007). Dr. Basta opined that because the work injury was responsible for only forty percent of the employee’s disability while sixty percent was attributable to the employee’s preexisting degenerative condition that was not job-related, the work injury was not a “major cause” under G. L. c. 152, § 1(7A). The administrative judge was under no obligation to adopt all of Dr. Basta’s opinion, especially because (as we discuss below) his conclusion was based on a legal misunderstanding of what qualifies as a “major cause.” See Clarici’s Case, 340 Mass. 495, 497 (1960) (single member and reviewing board free to adopt any portions of medical testimony they deem credible). In other words, as the board stated, it was not within the purview of the impartial medical examiner to define the legal scope of G. L. c. 152, § 1(7A). As the board determined, in light of Dr. Basta’s misunderstanding of the meaning of “major cause,” the administrative judge, who applied the correct standard to Dr. Basta’s assessment of the work injury as a forty percent cause, was warranted in concluding that the employee had met his burden of proof under § 1(7A). Indeed, where there are two identified causes of an employee’s disability, a forty percent cause may be a “major cause.” As such, the board’s decision was not arbitrary and capricious.
The self-insurer also claims that the board’s interpretation of [646]*646the phrase “major cause” was contrary to law. More specifically, the self-insurer claims that the board’s conclusion that a “major cause” can be less of a factor than the most important cause was erroneous and inconsistent with the plain meaning of the statute. We disagree.
In interpreting a statute, we look first to its language as “the principal source of insight into legislative intent.” Water Dept. of Fairhaven v. Department of Envtl. Protection, 455 Mass. 740, 744 (2010), quoting from Providence & Worcester R.R. v. Energy Facilities Siting Bd., 453 Mass. 135, 142 (2009). Where the meaning of the language is plain and unambiguous, we will not look to extrinsic evidence of legislative intent “unless a literal construction would yield an absurd or unworkable result.” Boston Hous. Authy. v. National Conference of Firemen & Oilers, Local 3, 458 Mass. 155, 162 (2010) (interpreting G. L. c. 150E, § 7[a], which was later amended by St. 2011, c. 198, § 1). See Halebian v. Berv, 457 Mass. 620, 630 (2010).
Section 1(7A) of c. 152, as amended by St. 1991, c. 398, § 14, provides in pertinent part:
“If a compensable injury or disease combines with a preexisting condition, which resulted from an injury or disease not compensable under this chapter, to cause or prolong disability or a need for treatment, the resultant condition shall be compensable only to the extent such compensable injury or disease remains a major but not necessarily predominant cause of disability or need for treatment.”
This provision of c. 152 “imposes a heightened proof of causation on an employee who claims benefits in a situation where the claimed injury may be owing to a resultant condition arising from a combination of injuries, the claimed compensable injury and a prior noncompensable injury. In those instances where the statute applies, as part of his proof of causation the employee must prove that the ‘compensable injury or disease remains a major but not necessarily predominant cause of disability or need for treatment.’ G. L. c. 152, § 1(7A).” (Emphasis supplied.) MacDonald’s Case, 73 Mass. App. Ct. 657, 659 (2009). The emphasized language of § 1(7A) is clear: “a major . . . cause” need not be the “major . . . cause.” See Commonwealth v. [647]*647Mandell, 61 Mass. App. Ct. 526, 528 (2004), quoting from Pyle v. School Comm. of S. Hadley, 423 Mass. 283, 285 (1996) (“[Wjhere the language of a statute is clear and unambiguous, it is conclusive as to legislative intent”). The Legislature’s choice of the article “a,” rather than “the,” to precede “major . . . cause” implies that there may be more than one major cause. See May’s Case, 67 Mass. App. Ct. 209, 212 (2006). In addition, the accompanying language (“but not necessarily predominant”) makes it explicit that a major cause need not be the superior or greatest cause, i.e., one that is more than fifty percent of the cause of the disability. If we were to adopt the self-insurer’s interpretation that a major cause cannot be less than the most important cause, we would render meaningless the Legislature’s choice of articles as well as the explicit caveat regarding predominance. This we cannot do. See Halebian v. Berv, 457 Mass, at 628 (“We give effect to each word and phrase in a statute, and seek to avoid an interpretation that treats some words as meaningless”). There was no error.2
Decision of reviewing board affirmed.