In the first withdrawal liability case to reach the Supreme Court in more than three decades, the Court will consider a narrow, but consequential, statutory question: May an actuary of a multiemployer pension plan adopt assumptions for calculating withdrawal liability after the statutory “measurement date” as long as the assumptions reflect the conditions on that date, or must the actuarial assumptions be adopted (and never altered) by the measurement date itself?

The answer will determine whether multiemployer pension plans have the flexibility to update actuarial assumptions after year end or whether those assumptions must be fixed at the close of the prior plan year, which provides more certainty to withdrawing employers.

The question arises in a case called M&K Employee Solutions, LLC v. Trustees of the IAM National Pension Fund.¹ Although the dispute may seem technical, its implications are substantial: The timing of actuarial assumptions, including when interest rates are calculated, can significantly impact the magnitude of withdrawal liability, often by hundreds of millions of dollars or more.

This blog post examines M&K in depth, analyzing the parties’ arguments as presented in the petition for certiorari and opposition and the D.C. Circuit’s decision. It also explores the meaning of “as of the measurement date,” and concludes by touching on what plans should do as they await the Supreme Court ruling.

How Is Withdrawal Liability Determined?

Congress enacted the Multiemployer Pension Plan Amendments Act of 1980 (MPPAA) to prevent employers from abandoning underfunded multiemployer pension plans and leaving remaining employers to shoulder the plan’s unfunded vested benefits (UVBs).² When an employer withdraws from an underfunded plan, it must pay its allocable share of the UVBs, which is calculated as the difference between the plan’s vested benefit obligations and its assets. This rule provides a legal basis for a plan to receive (often ongoing) compensation for continuing to provide promised benefits to employees of the withdrawing employer.³

Under MPPAA, withdrawal liability is determined “as of the end of the plan year preceding the plan year in which the employer withdraws.” This date (commonly called the measurement date) fixes the point at which the plan’s UVBs are valued. To calculate UVBs, actuaries must discount future benefit payments to present value, which requires selecting assumptions about future investment returns, mortality and other plan experience. This means that the actuary is estimating how those future benefit payments translate into today’s dollars. The statute imposes two substantive requirements on those assumptions: They must (1) be reasonable and (2) “offer the actuary’s best estimate of anticipated experience under the plan.”

However, the statute does not explicitly address when those assumptions must be adopted. This silence—and whether it implies a time-related limit—is at the center of the M&K case.

Relevant Facts

M&K Employee Solutions participated in the IAM National Pension Fund. The company and other employers withdrew during the 2018 plan year. The measurement date for calculating withdrawal liability was set at December 31, 2017. As of that date, the fund’s actuary had used a 7.5% discount rate to value the plan’s liabilities yielding UVBs of roughly $448 million.

On January 24, 2018 the fund’s trustees met with the actuary and adopted new actuarial assumptions, including a lower discount rate of 6.5%. A lower discount rate signifies that the plan expects to generate lower investment returns. Therefore, the change dramatically increased the plan’s UVBs to more than $3 billion, significantly increasing the withdrawal liability assessed to M&K and the other employers.

The employers challenged the assessment in arbitration, arguing that the Employee Retirement Income Security Act (ERISA) required the plan to use assumptions adopted by the 2017 measurement date. The arbitrator ruled for the employers. The district court vacated the award and remanded, holding that post measurement-date assumptions could be permissible if they reflected information available as of the measurement date.⁴

Following an appeal by the employer, the D.C. Circuit affirmed the district court decision.⁵

The D.C. Circuit’s Holding

In its decision to affirm, the D.C. Circuit held that MPPAA permits actuaries to adopt assumptions after the measurement date, if those assumptions are based on information “as of” that later date. The court reasoned that the law requires assumptions to reflect the actuary’s “best estimate” as of the measurement date, and that it would be illogical to force actuaries to make that estimate before they have complete year-end information.

The court rejected the employers’ argument that the law imposes a strict timing requirement, emphasizing that Congress specified substantive requirements for assumptions but not temporal ones. It also noted that employers are safeguarded against manipulation because they may challenge unreasonable assumptions in arbitration. Courts review those findings under a standard that requires a clear preponderance of the evidence to support the arbitrators’ findings.⁶

While other circuit court rulings have touched on questions of timing and actuarial assumptions in withdrawal liability calculations,⁷ the D.C. Circuit’s reading in M&K is the first to squarely conflict with the Second Circuit’s decision in a 2020 case Nat’l Retirement Fund v. Metz Culinary Mgmt., Inc.⁸ That case held that plans may not change assumptions after the measurement date. In response, the Supreme Court agreed to hear the case to resolve this circuit court split.

The Employers’ Argument: “As of” Means “By”

The employers argue that by directing plans to calculate UVBs “as of the end of the plan year,” MPPAA freezes both the data and the assumptions as of that date.⁹ Because actuarial assumptions are embedded in the definition of UVBs, the employers claim that those assumptions must be the ones in effect on the measurement date.

They stress that Congress knew how to authorize the use of later-adopted assumptions but did not do so in the law. They provide tax law as a similar example, where valuation with an “as of” a date requires using contemporaneous assumptions.

They also emphasize the importance of predictability and fairness. Employers that are contemplating withdrawal rely on disclosed plan data, including the assumptions in place before the measurement date, when making their decision. The employers argue that allowing plans to adopt new assumptions after a withdrawal enables retroactive manipulation that can inflate liability and game the system. This is the reason they argue that the fund increased liability from $448 million to $3 billion in UVBs after January 24, 2018.

Finally, the employers warn that the D.C. Circuit’s standard is unworkable, requiring litigation over subjective questions like what an actuary “knew” as of a date in the past. They claim that Congress could not have intended to hinge withdrawal liability on these uncertainties.

The Fund’s Argument: “As of” Means “With Reference To”

The fund responds that ERISA imposes no timing requirement for adoption of assumptions. Instead, the fund argues that MPPAA requires assumptions to represent the actuary’s “best estimate” as of the measurement date.¹⁰ Forcing actuaries to set assumptions before they have completed year-end data, the fund contends, would undermine the “best estimate” mandate.

The fund also notes that Congress included timing language elsewhere in ERISA but omitted it here, suggesting that no time-related limit was intended. And it argues that because the statute allows the parties to seek arbitration and can contest arbitrators’ findings, employers are protected from unreasonable assumptions without imposing a rigid deadline.

In response to the employers’ assertion that withdrawal liability assumptions should be predictable so employers can calculate their liabilities when considering withdrawal, the fund states that employers can never know their exact withdrawal liability in advance, because liability depends on plan data and actuarial decisions that are unavailable until after year end. Even under the rule that the employers support, employers would not learn the selected assumptions until after withdrawal. Thus, they argue that the employers have overstated their claims of unpredictability.

Finally, the fund characterizes the circuit split as narrow and tolerable because only two circuits have addressed the question directly, and forum-selection clauses in plan documents often dictate where cases must be heard.¹¹ These clauses would prevent parties from choosing a circuit that is more favorable to them because the plan documents would dictate where the case would be heard.

The Meaning of “As of the Measurement Date”

The heart of the dispute is the meaning of the phrase as of the measurement date.¹² The statutory phrase can have two interpretations:

1. Fixed date (M&K employers’ reading): Everything—data and assumptions—must be set on the measurement date itself. The phrase acts as a cutoff and no changes can be made after that date.
2. Reference point (the fund’s reading): The phrase defines the conditions to be measured, not the date of adoption. The actuary may adopt or amend assumptions later, so long as they describe reality on the measurement date.

Both points of view have support from the law, other court rulings and past policies.

The Impact of Timing on “Best Estimates”

An actuary’s “best estimate” based on investments, industry activity and plan demographics can change dramatically over time. It can also change due to other factors. The following hypothetical examples demonstrate how timing can change estimates.

Example 1: A Stock Market Crash

An employer withdraws from a fund on January 1, 2024 with an “as of” measurement date of December 31, 2023. If a market crash occurs in late December, audited data (reflecting the changes in the investment) may be unavailable until midyear. Actuaries are unlikely to view a withdrawal liability estimate that used assumptions based on a market environment no longer in place as their “best estimate.”

Under the reasoning in a prior withdrawal liability case, Sofco Erectors, Inc. v. Trustees of the Ohio Operating Engineers Pension Fund,¹³ the withdrawal liability assumptions must reflect the plan’s expected investment returns. It would seem that the assumptions based on audited data but not available until midyear reflecting the crash and yielding a higher, but possibly more accurate, withdrawal liability would be more consistent with the plan’s expected returns. Under the M&K employers’ view, the hypothetical plan must adopt actuarial assumptions by December 31 because of what they see as timing limitations in ERISA. Using this date would likely understate liability.

Example 2: A Change in Actuaries

An employer contemplates withdrawal in 2025 and on December 31, 2024, the plan’s actuary has a 7.25% discount rate. In March 2025, the trustees hire a new actuary who lowers the rate to 5.5%, dramatically increasing liability. This new actuary is still basing their assumptions on data that was available on December 31, 2024, but was more conservative in their estimates. This fact pattern is very similar to the facts in Metz, where the court concluded that the plan could not change assumptions even though the new assumptions were derived from data developed as of the measurement date, and the new actuary would not consider 7.25% to be their best estimate. However, if a plan can change actuaries to obtain the “best estimate” it needs, withdrawing employers may worry that plans will game the system to impermissibly raise the liability owed to the plan.

Conclusion: Multiemployer Plan Considerations

It is not clear how the Supreme Court will rule in the M&K case. The Court agreed to hear the case on June 30, 2025 and as of October 22, 2025, the case is in the briefing phase. Oral arguments are generally scheduled between October 2025 and April 2026 with many experts assuming they will be heard sometime between January and March 2026 with a decision next summer. 

In the meantime, plans should continue to document the “as of” basis. If the plan is adopting assumptions after the year end, it should maintain contemporaneous records showing that the assumptions reflect conditions as of the measurement date. These records should include investment data, economic forecasts, and trustee meeting minutes.

At first glance, M&K Employee Solutions appears to raise a narrow question of actuarial timing. But its implications reach far beyond the calendar. If the Supreme Court sides with the employers plans nationwide will need to fix withdrawal liability assumptions by the measurement date, potentially limiting their ability to incorporate the most accurate data. If it sides with the fund, plans will gain flexibility, but employers will face increased unpredictability and potential exposure to increased withdrawal liability.

Either outcome will ripple through withdrawal liability practice, influencing actuarial governance, litigation strategies and collective bargaining dynamics.

1. 92 F.4th 316 (D.C. Cir. 2024), cert. granted, No. 23-1209 (U.S. June 30, 2025), ↩︎
2. See Pension Benefit Guar. Corp. v. R.A. Gray & Co., 467 U.S. 717 (1984) (finding that the retroactive application of the withdrawal liability rules within MPPAA does not violate due process). ↩︎
3. 29 U.S.C. §§ 1381(a), 1393(c). ↩︎
4. Trs. of IAM Nat’l Pension Fund v. M&K Emp. Sols., LLC, No. 21-cv-2152, 2022 WL 4534998 (D.D.C. Sept. 28, 2022). ↩︎
5. M&K, 92 F.4th at 326. ↩︎
6. Id. at 329; see 29 U.S.C. § 1401(a)(3)(B). ↩︎
7. Cases include the Sixth Circuit’s Sofco decision, the D.C. Circuit’s UMWA v. Energy West, and the Ninth Circuit’s MNG Enterprises. ↩︎
8. Nat’l Retirement Fund v. Metz Culinary Mgmt., Inc., 946 F.3d 146 (2d Cir. 2020), ↩︎
9. Brief for the Petitioners at 17–18, M&K, No. 23-1209 (U.S. Aug. 28, 2025). ↩︎
10. Brief in Opposition at 22–23, M&K, No. 23-1209 (U.S. July 12, 2025). ↩︎
11. Id. at 11–13. ↩︎
12. 29 U.S.C. § 1391(b)(2)(E)(i). ↩︎
13. Sofco Erectors, Inc. v. Trustees of the Ohio Operating Engineers Pension Fund, ↩︎