BOTSFORD, J.

This case concerns the modification of a child support order, and in particular, the standard to be used by a Probate and Family Court judge in reviewing a complaint for modification. The child support order at issue is included in a 2008 divorce judgment that requires the defendant, Richard Louis Morales (father),1 to make weekly child support payments to the plaintiff, Marlene Morales (mother), for support of their minor child. In 2009, approximately one year after the divorce judgment, and following the father's job promotion, the mother filed a complaint requesting the modification of the child support order to reflect the father's increase in income. Following a trial before a judge in the Probate and Family Court, the judge found that there was no "material and substantial change of circumstances and no modification [was] warranted," and dismissed the complaint.

We conclude that the trial judge, in ruling on the mother's modification complaint, erred in applying a standard requiring a material and substantial change in circumstances (material and substantial change standard) rather than the standard set forth in G. L. c. 208, § 28, as amended through St. 1998, c. 64, §§ 194, 195 (§ 28),2 which provides that a child support order shall be modified "if there is an inconsistency between the amount of the existing order and the amount that would result from application of the child support guidelines"3 (inconsistency standard). Accordingly, we remand for the judge to consider the child support modification request under the statutory inconsistency standard.

1. Background. The parties' child was born on August 4, 1998. The mother and father were divorced by a judgment of divorce nisi dated May 5, 2008, that granted shared legal custody of the child and physical custody to the mother. The judgment included a child support order directing the father to pay $172 per week in child support to the mother. In August, 2008, the father, a correction officer at the Massachusetts Correctional Institution at Shirley, received a promotion to the position of inner perimeter officer, resulting in an increase in his salary and his average weekly income from overtime. On April 29, 2009, the mother filed a complaint for modification of the child support order in the Probate and Family Court, claiming that the father's new position and increased salary had changed the circumstances underlying the original support order and requesting an increase in the amount of weekly support to reflect the amount required by application of the Massachusetts Child Support Guidelines (2009) (guidelines, or 2009 guidelines).

The modification complaint was tried in December, 2009. At the trial, in response to the suggestion of the mother's counsel that the guidelines permit the judge to consider overtime in calculating income, the judge stated: "Well, I don't. I just don't. So everybody should know that right up front. I do not include overtime." Following the trial, the judge dismissed the modification complaint on December 21, 2009, finding that the increase in the father's income was "not ... a material and substantial change of circumstances and no modification is warranted." Some months later, the judge issued her opinion containing findings of fact and conclusions of law in support of the previously entered judgment of dismissal. The judge found that the father's change in income, combined with a decrease in living expenses of the mother and an increase in living expenses of the father, did not amount to "a substantial or material change in circumstances" to warrant an increase in the father's child support obligation under § 28. The judge also declined to include the father's overtime pay in her calculation of his weekly income because "[overtime] is not always available to him and is not a requirement of his employment," and because the father had parenting responsibilities to other children.

The Appeals Court affirmed the dismissal in an unpublished memorandum and order issued pursuant to its rule 1:28. Morales v. Morales, 80 Mass.App.Ct. 1110 (2011). The court agreed with the judge that the material and substantial change in circumstances standard was the correct legal standard to apply and that she had correctly determined that there was no material change in circumstances. We granted the mother's application for further appellate review.

2. Discussion. The method for calculating and modifying child support orders is governed by statute and by the guidelines.4 General Laws c. 119A sets forth the Commonwealth's general policy governing child support enforcement.5

Section 13 (c) of that chapter provides:

G. L. c. 119A, § 13 (c). This same presumption is incorporated in other sections of the General Laws pertaining to child support orders that arise in different contexts, including § 28. See G. L. c. 208, § 28 (child support orders for children of divorced parents); G. L. c. 209, § 37 (orders for children of separated parents); G. L. c. 209C, § 20 (orders for children born out of wedlock). Because the order in this case was issued as part of a divorce judgment, § 28 governs. With respect to modifications of child support orders, § 28 provides in relevant part:

"We start with the proposition that where the statutory language is clear, the courts must give effect to the plain and ordinary meaning of the language." Victor V. v. Commonwealth, 423 Mass. 793, 794 (1996), and cases cited. The "plain and ordinary" meaning of the quoted language is that when a complaint seeking modification of a child support order is filed, modification is presumptively required whenever there is an inconsistency between the amount of child support that is to be paid under the existing support order and the amount that would be paid under the guidelines.7 See, e.g., Hashimi v. Kalil, 388 Mass. 607, 609 (1983), citing Johnson v. District Attorney for the N. Dist., 342 Mass. 212, 215 (1961) ("The word `shall' is ordinarily interpreted as having a mandatory or imperative obligation"). Nothing here or elsewhere in § 28 establishes a separate and additional requirement that the discrepancy or inconsistency between the existing order and the guidelines amount of child support result from a material and substantial change in circumstances.8 The quoted language embodies the inconsistency standard, and it properly governs consideration of a child support modification request to which § 28 applies.9

The judge did not apply the inconsistency standard in this case. Instead, although she cited § 28 as the basis for her decision, she applied the material and substantial change standard. This was error, and remand of the case is necessary to permit consideration of the mother's modification request under the appropriate legal standard. See, e.g., Smith v. McDonald, 458 Mass. 540, 550 (2010) (remanded for reconsideration of custody and visitation orders under correct legal standards). However, before discussing specific claims of the mother that may arise on remand, we consider the provisions in the 2009 guidelines governing modification requests, because, as the Appeals Court concluded, these provisions clearly purport to apply in the present case.

The 2009 guidelines state in pertinent part:

2009 guidelines III.A. These modification provisions appear to provide that any child support order less than three years old may be modified only if there has been either (1) a change in health insurance coverage, or (2) a material change in circumstances.10 In so limiting the availability of modification, the 2009 guidelines are themselves not consistent with the inconsistency standard set out in § 28, at least with respect to modification requests filed less than three years after the date of the original child support order.

This discrepancy between statute and guidelines may have a historical explanation. Under applicable provisions of Federal statutes, a State's receipt of certain Federal grants and reimbursements is conditioned on the State's creation of guidelines for child support that meet specific statutory and regulatory criteria.11 The Family Support Act of 1988, an amendment to Title IV, Part D, of the Social Security Act, 42 U.S.C. §§ 651-669b (2006 & Supp. V 2011) (Title IV-D), mandated periodic review and adjustment of child support orders pursuant to a State's child support guidelines. See Pub. L. 100-485, 100th Cong., 2d Sess., 102 Stat. 2343 (1988). The Federal implementing agency, the United States Department of Health and Human Services, interpreted the 1988 statutory amendment to preclude a State's use of the material and substantial change standard in the context of modification of a child support order.12 Following the passage of the Family Support Act of 1988 and implementing regulations, the Massachusetts Legislature amended § 28 to provide that child support modifications would thereafter be governed by the inconsistency standard.13 Although there have been subsequent amendments to Title IV-D that reflect yet additional changes to the Federal legal standard applicable to support order modifications,14 the inconsistency standard in § 28 has not been amended to take these Federal statutory amendments into account.15 Accordingly, we are bound to apply the provisions of § 28 as written. Commonwealth v. Russ R., 433 Mass. 515, 520 (2001), quoting Commissioner of Revenue v. Cargill, Inc., 429 Mass. 79, 82 (1999) ("Where, as here, the language of the statute is clear, it is the function of the judiciary to apply it, not amend it").

3. Remaining issues. We turn to other issues raised by the mother that may arise on remand.16

The mother contends that the stated position of the judge — that she never considers a payor's overtime income when calculating a child support order — constitutes an error of law. The 2009 guidelines define income as "gross income from whatever source" including income from "salaries, wages, overtime and tips" (emphasis supplied). Guidelines I.A. While the guidelines permit a judge to "disregard[] income, in whole or in part, from overtime," the judge must first give "due consideration ... to certain factors including but not limited to the history of the income, the expectation that the income will continue to be available, the economic needs of the parties and the children, the impact of the overtime on the parenting plan, and whether the extra work is a requirement of the job." Guidelines I.B.

In her findings, the judge stated that although the "[f]ather averages overtime income of $100.00 per week ... [t]he court declines to include this amount in his weekly income as it is not always available to him and is not a requirement of his employment" and because of his parenting responsibilities to other children. Considered in isolation, these reasons may well comply with the 2009 guidelines, although arguably the guidelines embrace a more generous approach to the inclusion of overtime in income calculation than the one adopted by the judge.17 However, where the judge announced at the beginning of the trial that "everybody should know that right up front. I do not include overtime," we are concerned that the judge created at least an appearance that even though she later made findings, she did not give a fair consideration to the facts of the case in deciding to exclude overtime. An inflexible rule that overtime not be included in the calculation of income would be at odds with the 2009 guidelines, which clearly mandate consideration of specific factors prior to a decision to disregard overtime income. If the father's receipt of overtime pay is again at issue on remand, the guidelines' mandate must be applied.

The mother also argues that the judge's findings with respect to the amount by which the father's income increased are clearly erroneous. The mother's position is that the judge should have based her income calculations on the father's base pay, overtime, and roll call and longevity pay as listed on the father's sworn 2008 and 2009 financial statements and as to which the father testified at trial. Instead, the mother argues, the judge erroneously calculated the father's increase in income based solely on the increase in his base hourly wage rate between 2008 and 2009 that she calculated using two individual paystubs that were attached to the father's financial statements. The judge did not explain why she limited her calculation to the hourly wage information derived from the two paystubs instead of the sworn financial statements, or why she did not consider the roll call and longevity pay components; we are not able to discern the judge's rationale from the record itself. On remand, the reasons supporting the methods used by the judge to determine the amount of each party's income should be supplied.

4. Conclusion. The matter is remanded to the Probate and Family Court for further proceedings consistent with this opinion.

So ordered.