Boyle, J.
We are asked to determine whether the Michigan Education Association (MEA) committed an unfair labor practice under MCL 423.210(3)(c); MSA 17.455(10)(3)(c) and MCL 423.216; MSA 17.455(16) of the public employment relations act (pera) by unilaterally implementing a midterm modification of a collective bargaining agreement.
We affirm the decision of the Court of Appeals, which upheld the conclusion of the Michigan Employment Relations Commission (merc) that the Michigan Educational Special Services Association (messa) was an agent of the mea and that the increase in the lifetime maximum health care benefit was a violation of the statute.
FACTS1
The Michigan Educational Special Services Association is a Michigan nonprofit, nonstock corporation. Its purpose is to provide various types of insurance benefits to its membership. The MESSA bylaws restrict [543]*543membership to Michigan Education Association2 members and certain specified categories of employees with past or present ties to an mea bargaining unit, including employees of the MESSA itself.
The MESSA is not an insurance company; rather, it is a policyholder that contracts with a qualified insurance company to underwrite its various plans. Blue Cross/Blue Shield of Michigan is currently the underwriter for MESSA health insurance plans. Blue Cross and the messa share responsibility for administering the plans. The MESSA qualifies as a third-party administrator under the Insurance Code.
The MESSA is managed by a board of trustees consisting of thirteen persons, all of whom must be messa members. Under the bylaws, six trustees must be elected from and by the MEA board of directors, and five must be elected by the messa voting members. Voting members of the messa consist of the incumbent trustees and officers of the MESSA and incumbent members of the mea board of directors, who are members of the messa. The final two trustees are the president and vice president of the mea. The same person serves as the executive director of the mea and the executive secretary of the messa. The messa has its own executive director.
The mea “markets” the messa insurance by persuading members of its constituent units and local affiliates of the superiority of the messa plans and by negotiating these plans into collective bargaining agreements reached with employers. The Mea UniServ directors are largely responsible for bargaining and [544]*544for successfully negotiating contracts, including messa plans. The UniServ directors are evaluated in part by how well they achieve negotiation of the messa products into collective bargaining agreements, and the mea is required to use its best efforts to negotiate MESSA benefits into collective bargaining agreements. The mea provides the MESSA with legislative, lobbying, and research services. The messa currently leases from the MEA the land on which its building sits, and the current MESSA building is adjacent to mea headquarters. The mea and the messa have a written agreement under which the messa compensates the mea for these services. For the fiscal year 1989-90, the messa paid the mea $1,400,424.
The mea is the authorized collective bargaining agent for the St. Clair School District’s teaching employees. Messa health coverage for at least some employees has been included in the contract between the mea and the school district since approximately 1967. Article IX of the parties’ collective bargaining agreement for the period ending June 30, 1991, reads:
The District agrees to pay premiums for health insurance for each teacher through a carrier to be determined by the Board. For the (3) year period 1988-89 through 1990-91, the District agrees to provide Messa Super Med II for 1988-89 and Messa Super Care II[3) for 1989-90 and 1990-91.
Total health insurance payments for any teacher will not exceed the actual cost of the messa plan herein outlined. There will be no supplemental payments by the Board. The payments will be the full premium amounts for the messa [545]*545plan outlined herein for single, two persons, or full family coverage.
Effective July 1, 1990 and until a successor Agreement is reached, the obligation of the Board to pay health insurance premiums shall not exceed the Board’s base premium amount for the 1990-91 insurance year, July 1, 1990 to June 30, 1991. If health insurance premiums effective July 1,1991, exceed the Board’s base premium for the 1990-91 insurance year, the excess amounts over the individual employee’s premium cost, shall be paid in full by the individual employee by way of payroll deduction.
In addition, the school district signed an “Employer Participation Agreement For Negotiated Group Benefit Programs” with the MESSA in February, 1990. The participation agreement includes the following paragraph:
If accepted for participation, the Employer agrees to be bound by the terms of the Trust, the Equitable group insurance policy(ies) and the. Bcbsm Group Operating Agreement(s) and certificates issued to messa. Copies of the above may be examined by any Participating Employer during the regular business hours at the office of messa in East Lansing, Michigan.
In 1985, the MESSA changed its underwriter of health insurance from the Equitable Insurance Company to Blue Cross. The MESSA took the position that, because coverage in each bargaining unit was pursuant to a collective bargaining agreement, each employer and local association would have to negotiate and agree to the change in carriers. The reason for this position was that the messa hoped these negotiations would force employers to pass the savings resulting from the change along to employees, rather than permit employers to enjoy the reduced premiums. The MEA and the MESSA asserted that the messa hoped such [546]*546negotiations would lead to purchases of other messa insurance products.
Aside from this instance, however, the record indicates that the messa, by action of its trustees, made minor plan changes to insurance benefits without giving prior notice or an opportunity to renegotiate the change to employers. One change, other than the increase in the lifetime maximum made unilaterally by the MESSA during the term of the parties’ 1989-91 contract, was the addition of a wig benefit for cancer patients.
On March 9, 1990, the messa staff presented the trustees with a proposal to raise the lifetime maximum on all messa plans to $2,000,000. Reasons cited to the trustees were that several members were then approaching the maximum, the effect of inflation in medical costs, and that competing plans had raised their máximums. Beverly Wolkow, mea executive director and the executive secretary of the messa, argued against the change on the basis that it would make messa insurance more difficult to sell to employers at the bargaining table. However, the change was approved by the trustees.
The school district was notified of this change by letter dated May 8, 1990, which also described rate changes to be effective July 1, 1990.4 The school district did not make a demand to bargain, but filed an unfair labor charge on the basis that the change was an accomplished fact.
[547]*547After the original hearing referee had heard the case, a request by the school district to reopen the record in order to add the messa’s affiliation/ disaffiliation policy and subsequent testimony to the record was granted by a subsequent hearing referee. The affihation/disaffiliation policy referenced in the messa bylaws and adopted by the board of trustees in March, 1982, provides that individual membership in messa health insurance plans terminates for employees eligible for membership in the mea if the employees affiliate with a labor organization other than the mea. This information had not been available to the school district before the original hearing because commission procedure does not allow for prehearing discovery. The affihation/disaffiliation pohcy was considered material to the question whether there was an agency relationship between the MESSA and the MEA.5
PROCEDURE
The hearing referee found that the MESSA was an agent of the mea and that the unilateral change in the benefit maximum was an announced, not proposed, change in the contract, resulting in a midterm modification of the collective bargaining agreement. The hearing referee also held that the charging party, the school district, had waived the right to bargain with respect to benefit changes because under the contract, the employer participation agreement bound the employer to the language of the insurance certificates issued to the messa, thus allowing the messa to modify or discontinue the group plan. The merc affirmed the ruling of the hearing referee6 with the exception that [548]*548it disagreed that there was a waiver of the right to bargain over a modification or unilateral action by respondents MBA and messa.7
[549]*549The Court of Appeals affirmed the MERC decision, 218 Mich App 734; 555 NW2d 267 (1996), holding that, in bargaining for the Messa Super Care II policy, the Charging Party bargained for the specific contents of the policy when the collective bargaining agreement was entered and that the school district was denied its opportunity to renegotiate before unilateral change [550]*550of the policy was effected, thus resulting in an unfair labor practice. We granted leave to appeal. 456 Mich 901 (1997).
i
The PERA governs labor relations in public employment. It imposes a duty of collective bargaining on public employers, unions, and their agents, MCL 423.210; MSA 17.455Q0).8 Violations of § 10 of the pera are deemed unfair labor practices under MCL 423.216; MSA 17.455(16) remediable by the Michigan Employment Relations Commission. We have interpreted § 16 as vesting the merc with exclusive jurisdiction over unfair labor practices. Detroit Bd of Ed v Parks, 417 Mich 268, 283; 335 NW2d 641 (1983). By statute, the employer and the representative of the employees are required to bargain for wages, hours, and other terms and conditions of employment, MCL 423.215; MSA 17.455(15),9 which constitute mandatory [551]*551subjects of collective bargaining. Pontiac Police Officers Ass’n v Pontiac (After Remand), 397 Mich 674, 679; 246 NW2d 831 (1976).
Mandatory subjects of collective bargaining are comprised of issues that “settle an aspect of the relationship between the employer and employees,” Allied Chemical & Alkali Workers of America v Pittsburgh Plate Glass Co, 404 US 157, 178; 92 S Ct 383; 30 L Ed 2d 341 (1971), and include, but are not limited to, terms and conditions of employment concerning hourly, overtime, and holiday pay, work shifts, pension- and profit sharing, grievance procedures, sick leave, seniority, and compulsory retirement age. Detroit Police Officers Ass’n v Detroit, 391 Mich 44; 214 NW2d 803 (1974). Health insurance benefits are mandatory subjects of bargaining. Port Huron Ed Ass’n v Port Huron Area School Dist, 452 Mich 309, 317, n 12; 550 NW2d 228 (1996).10 Although the PERA does not currently allow labor to bargain for “[w]ho is or will be the policyholder of an employee group insurance benefit,”11 this does not “affect the duty to [552]*552bargain with respect to types and levels of benefits and coverages for employee group insurance.” MCL 423.215(3)(a); MSA 17.455(15)(3)(a).12 Under subsection 15(3)(a) of the pera, changes in levels of benefits must be bargained by the employer and the employees’ bargaining representative.13
The existing three-year collective bargaining agreement between the school district and the mea is the manifestation of the parties’ discharge of their statutory obligations. That is, “ ‘[a]fter the parties have met in good faith and bargained over the mandatory subjects placed upon the bargaining table, they have satisfied their statutory duty.’ ” Port Huron, supra at 322. The school district approved the messa as the policyholder and agreed specifically to the Messa Super Med II and Super Care II programs as they existed at the time of the agreement, which was [553]*553memorialized in a written contract14 confirmed by both parties. By unilaterally modifying the existing contract in midterm without the agreement of the school district, respondent MEA, committed an unfair labor practice.15
n
As an initial matter, this Court has acknowledged the expertise and judgment possessed by the MERC in the labor relations arena. We are also mindful of the fact that
[o]ur review of the commission’s decision is circumscribed by the statutory mandate that factual findings of the commission are conclusive if supported by competent, material, and substantial evidence on the record considered as a whole. MCL 423.216(e); MSA 17.455(16)(e), Const 1963, art 6, § 28. Review of factual findings of the commission must be undertaken with sensitivity, and due deference must be accorded to administrative expertise. Reviewing courts should not invade the exclusive fact-finding province of administrative agencies by displacing an agency’s choice between two reasonably differing views of the evidence. MERC v Detroit Symphony Orchestra, 393 Mich 116, 124; 223 NW2d 283 (1974). [Amalgamated Transit Union, Local 1564, AFL-CIO v Southeastern Michigan Transportation Authority, 437 Mich 441, 450; 473 NW2d 249 (1991).]
The principal focus of this controversy centers on the commission’s determination that the messa is an agent of the mea. The findings of fact made by the [554]*554merc are conclusive if supported by substantial, material, and competent evidence on the record as a whole. Independent review of the record supports the following facts.16
The messa is not a labor organization, but is an independent coiporate subsidiary of the MEA and a member of a family of organizations formed by and affiliated with the MEA, the collective bargaining agent for the school district’s employees. The messa’s original articles of incorporation indicate that it was formed to “benefit . . . members of the Michigan Education Association . . . .” The messa acts as an insurance agent to provide various forms of insurance for members of the mea and acts as a third-party administrator under the Insurance Code. The MESSA and the MEA have an interlocking board of trustees consisting of thirteen persons, all members of the MESSA. The MESSA bylaws require that six trustees must be elected from and by the mea board of directors and five others are elected by the messa. The final two trustees are the president and vice president of the MEA. Thus, the mea holds a majority voting position on the messa board of trustees. Additionally, the positions of executive director of the mea and the executive secretary (or chief executive officer) are filled by the same person. The performance of the executive director of the MEA is evaluated by the MEA board of directors, in part on the basis of how well the director controls and monitors the MESSA.
The MEA is the exclusive agent for the messa and markets its products during the collective bargaining [555]*555process. The messa pays the mea for its services, which contractually obligates school districts to provide messa products. In 1990, the amount paid was over one million dollars. The mea professional staff has a group of employees called UniServ directors, business agents for the mea who bargain for and administer contracts for mea members. UniServ directors are evaluated in part on the basis of how well they achieve negotiation of messa products into collective bargaining agreements. The mea is required to use its “best efforts” in obtaining employee participation in the MESSA, and the UniServ staff is required to provide the messa with a list of presidents and negotiators, copies of fringe benefit contract language, and involve the messa in meetings where bargaining guidelines are established. All school employee members of the messa are either mea members or employees of school districts represented by the mea. Because the messa is solely dependent on the mea to bargain for a role in insurance coverage, its agreement with the MEA requires the MEA to actively inform and consult the messa concerning collective bargaining activities for insurance benefits. Numerous other documents support the fact that the messa played a supporting role, directly or indirectly, in the collective bargaining process.
When the messa board decided to change underwriters of their health insurance program from Equitable to Blue Cross/Blue Shield, the messa required that each employer send a request to the messa approving the change. If the insurance coverage was pursuant to a collective bargaining agreement, the MESSA required a written notification jointly signed by the employer and the local bargaining agent. This action triggered [556]*556collective bargaining in each unit, and the savings to the employer resulting from the underwriter change gave the union the opportunity to bargain with the employer for the use of the savings. By this decision, the messa became part of the bargaining process and significantly advanced the bargaining interests of the mea by enhancing the labor organization’s ability to obtain additional benefits pursuant to a new collective bargaining agreement.
Furthermore, the messa’s affiliation/disaffiliation policy directly advanced the interests of the mea in derogation of the interests of the messa membership. This policy requires that MESSA members who choose to disaffiliate from the mea by choosing another union to represent their labor interests lose MESSA health insurance, despite the fact that it is in the interest of the MESSA to retain membership and in the interest of the messa members to retain their choice of health insurance previously obtained through the bargaining process. Testimony by an ex-executive director of the messa explained that, because the mea sponsored the MESSA, the messa needed to maintain a good relationship with the mea to assist in bargaining messa’s benefit products. Without the disaffiliation policy, movement away from an organization that helped the messa could be encouraged. The mea/messa relationship was, as frankly noted by the ex-director, one where “We scratched each other’s backs . . . .”
“When there is a disputed question of agency, if there is any testimony, either direct or inferential, tending to establish it, it becomes a question of fact . . . .” Miskiewicz v Smolenski, 249 Mich 63, 70; 227 NW 789 (1929). In this case, the factual determinations were within the province of the merc, the des[557]*557ignated trier of fact. Our statutory mandate, MCL 423.216(e); MSA 17.455(16)(e), provides that the commission’s factual findings are deemed conclusive “if supported by competent, material, and substantial evidence . . . .” We review the merc’s factual findings with the deference due administrative expertise and accept findings of fact if supported by substantial evidence.17 Substantial evidence is “such evidence as a reasonable mind will accept as adequate to justify conclusion” and requires judicial review of the whole record. This entails a degree of qualitative and quantitative evaluation of the evidence considered by the agency. MERC v Detroit Symphony Orchestra, supra at 122.
m
Under the common law of agency, in determining “[w]hether an agency has been created,” we consider “the relations of the parties as they in fact exist under their agreements or acts” and note that in its broadest sense agency “includes every relation in which one person acts for or represents another by his authority.” Saums v Parfet, 270 Mich 165, 170-171; 258 NW 235 (1935). We further recognized in Saums that “[t]he characteristic of the agent is that he is a business representative. His function is to bring about, modify, affect, accept performance of, or terminate contractual obligations between his principal and third persons.” Id. at 172. Also fundamental to the existence of an agency relationship is the right to [558]*558control the conduct of the agent, Capitol City Lodge No 141, FOP v Meridian Twp, 90 Mich App 533, 541; 282 NW2d 383 (1979), with respect to the matters entrusted to him. See Int’l Longshoremen’s Ass’n, AFL-CIO v NLRB, 312 US App DC 241, 249; 56 F3d 205 (1995), citing 1 Restatement Agency, 2d, § 14, p 60, and cases18 applying this principle.19
[559]*559We have long recognized that the pera is patterned after the National Labor Relations Act. The construction of analogous provisions of the nlra provides guidance in interpreting the pera. Goolsby v Detroit, 419 Mich 651, 660, n 5; 358 NW2d 856 (1984); Gibraltar School Dist v Gibraltar MESPA, 443 Mich 326, 335; 505 NW2d 214 (1993). Accordingly, we have not previously determined the meaning of the word agent as used in this context.20 We conclude that the Legislature did not intend an expansive definition of agency in the pera, but, rather, adopted the common-law principles of agency in use in federal labor law.21 [560]*560Regarding both management and labor, the NLRB and federal courts have long held that the common law of agency governs the question who acted for whom for purposes of determining “culpability”22 under the [561]*561nlra. NLRB v Int’l Longshoremen’s & Warehousemen’s Union, 283 F2d 558, 563 (CA 9, 1960).23
We agree with both the hearing referee and the MERC that the facts support a finding that more than a “mere” agency relationship exists between the messa and the mea. The mea created the messa as a subsidiary corporation24 and was the exclusive sponsor for messa products other than to the messa’s own employees. Thus, the mea and the messa were bound by a formal affiliation and common agreement. Under the messa bylaws, mea members had majority control of the messa board that made the decision to increase the benefit level. This advanced the interests of the mea and its members because increasing the benefit level without having to renegotiate the modification placed the mea in a stronger position after the expira[562]*562tion of the current contract to bargain from the position of an accomplished change, a “done deal,” so to speak. The MESSA also had substantial input into the collective bargaining process and had an agreement with the mea to keep it involved and informed concerning marketing of its products to the MEA members.
The members of the messa, except for employees of the messa itself, were all required to be members of the mea or employed in mea controlled school districts. If the mea no longer represented the employees of a district, the disaffiliation policy dictated loss of messa membership and benefits, thus the mea controlled both the messa membership and access to MESSA benefits. This consequence advanced the interests of the MEA at the expense of its own interests and the interests of its members. We agree that these facts indicate a degree of control over the MESSA by the MEA and substantial joint interests advanced by their reciprocal agency arrangements.
While limited deference is accorded to the commission’s analysis concerning common-law agency principles,25 in this case, we agree with and affirm the legal rulings of the MERC. The facts support a finding of agency on the basis of the common law of agency as [563]*563developed both by Michigan courts and federal administrative and judicial precedent.
IV
Section 8 of the nlra, 29 USC 158(b)-(d)26 is analogous to the pera provisions governing unfair labor [564]*564practices in Michigan. Subsection 8(d) of the nlra describes the duties of both employer and union when a party proposes a midterm modification27 of an [565]*565existing agreement and includes notification, negotiation, and a prohibition of strikes and lockouts until the existing agreement expires. It also grants to either party the right to refuse to discuss or agree to a midterm modification of the contract and rejects the duty to bargain over a modification of a collective bargaining agreement before it expires. While the parties may agree to negotiate a change midterm during the effective term of the written agreement, bargaining is not required and is purely voluntary. Allied Chemical Workers, supra at 183.28 Failure to meet the statutory requirements results in a violation of § 8 [566]*566resulting in an unfair labor practice by the party implementing the modification. The hearing referee observed, and we agree, that the same principle applies although the PERA has no explicit provision akin to the nlra, § 8.
The lead federal case on this doctrine relies primarily on Section 8(d) of the National Labor Relations Act, which permits a party to refuse to bargain on a mid-term modification. Oak Cliff-Golman Baking Company, 207 NLRB 1063 (1973), enfd 505 F2d 1302; 85 LRRM 1035 (CA 5, 1974), cert den 423 US 826 (1975); 90 LRRM 2614 (1975). Pera contains no similar provision, but the principle applies. ... To treat this solely as a “unilateral change” and to fail to distinguish these theories of violation of the bargaining duty would mean that a party might give timely notice, bargain in good faith to impasse and then “properly” implement a “unilateral” change in an existing contract. Furthermore, the right to refuse to bargain in mid-term may apply only where the subject has been integrated and embodied in the agreement or discussed in negotiations. . . . Here the modification consisted of a change of one digit in a figure incorporated into the contract. Clearly, this is a contract modification situation.
We affirm the conclusion that the benefit change in the health insurance contract was properly designated a midterm modification29 of the agreement.
Once agreement is reached, the terms of the written bargaining agreement are preserved and neither management, Int'l Union v NLRB, 246 US App DC [567]*567306, 310; 765 F2d 175 (1985),30 nor labor, Teamsters Cannery Local 670 v NLRB, 856 F2d 1250, 1257 (CA 9, 1988)31 may unilaterally modify the agreement without the consent of the other party. As we recognized in Port Huron, supra at 324, when a matter is “covered by” the provision of a collective bargaining agreement, the parties have created a set of enforceable rules for themselves.32 In short, both the nlra and the PERA provide “only a process by which the parties might reach agreement, the power to agree to a proposal remains with each party.” Gibraltar School Dist, supra at 341-342. The statutory process authorized for midterm modification of the existing bargaining agreement directs that there can be no unilateral change without negotiation and agreement of the parties. As we observed in Amalgamated Transit, supra at 450:
[568]*568Thus, while the parties may by contract agree to grant the right to take unilateral action, it is well established that neither party may take unilateral action on such a subject unless it either has satisfied the statutory obligation or has been freed from it.
The terms of the mea contract with the school district contained the specific health care coverage and benefit levels bargained for33 by the parties as required by statute. This mandatory term was “covered by” the contract. Because bargaining is not required during the term of the contract, any change is voluntary. A midterm modification would require the mea and the MESSA to negotiate an agreement with the school district to implement the change, just as the school district would have had to reopen negotiations to decrease the benefit level. The messa did not notify or negotiate an agreement with the school district before implementing the change. The messa’s board of trustees approved the increase of a maximum lifetime benefit to two million dollars on March 9, 1990, and informed the school superintendents of the change by letter dated May 8, 1990.34 The [569]*569hearing referee properly determined that the change was an “announced change” and not a “proposed change.”35 Additionally, as recognized by the hearing referee, the consequence of the notice has little legal effect under a midterm modification theory because change is not permissible absent mutual consent. Silence by the school district in this situation could “preclude any change in the contract.”
The commission also reviewed the mea assertion that the school district had waived its right to bargain by executing the collective bargaining agreement containing an “Employer Participation Agreement.” The claim was that this agreement bound the employer to the insurance certificate issued to the messa and that in the certificate the messa reserved the right to modify or discontinue the group program at any time. Disagreeing with these facts, the merc found that this language was not contained in the insurance certificate but, rather, in a pamphlet distributed to employees explaining the health care benefits provided by the messa. The merc concluded that the school district did not bind itself or surrender its bargaining right through the pamphlet language. The parties do not dispute this finding and the record supports it. We affirm the commission’s conclusion.
[570]*570The mea also argued that the district had waived its right to bargain by the past practice of acceding to other unilateral changes made by the messa in the health care contract. One example of change cited by the MESSA that occurred during the contract period was the decision to cover the purchase of wigs for hair loss resulting from chemotherapy. It is well established that past practice may create a term or condition of employment not incorporated in the collective bargaining agreement, thus waiving the statutory obligation. Port Huron, supra at 325. In this case, however, the term or condition was specifically “covered by” the agreement.
As recognized by the MERC and confirmed in Port Huron, supra at 319, there is a difference between whether a subject is covered by a collective bargaining agreement and whether the right to bargain about a mandatory subject has been waived. We quoted with approval the opinion of Judge Harry T. Edwards, a noted labor law scholar describing the distinction.
A waiver occurs when a union knowingly and voluntarily relinquishes its right to bargain about a matter; but where the matter is covered by the collective bargaining agreement, the union has exercised its bargaining right and the question of waiver is irrelevant.
* * *
When parties bargain about a subject and memorialize the results of their negotiation in a collective bargaining agreement, they create a set of enforceable rules — a new code of conduct for themselves — on that subject. Because of the fundamental policy of freedom of contract, the parties are generally free to agree to whatever specific rules they like, and in most circumstances it is beyond the competence of the Authority, the National Labor Relations Board or the courts to interfere with the parties’ [571]*571choice. ... On the other hand, when a union waives its right to bargain about a particular matter, it surrenders the opportunity to create a set of contractual rules that bind the employer, and instead cedes full discretion to the employer on that matter. For that reason, the courts require “clear and unmistakable” evidence of waiver and have tended to construe waivers narrowly. [Dep’t of Navy v Federal Labor Relations Authority, 295 US App DC 239, 248; 962 F2d 48 (1992), quoted in Port Huron, supra at 319.]
As we explained in Port Huron, supra at 325-327:
In order to create a term or condition of employment through past practice, the practice must be mutually accepted by both parties. Amalgamated, supra at 454. Where the collective bargaining agreement is ambiguous or silent on the subject for which the past practice has developed, there need only be “tacit agreement that the practice would continue.” Amalgamated, supra at 454-455. However, where the agreement unambiguously covers a term of employment that conflicts with a parties’ past behavior, requiring a higher standard of proof facilitates the primary goal of the PERA — to promote collective bargaining to reduce labor-management strife. A less stringent standard would discourage clarity in bargained terms, destabilize union-management relations, and undermine the employers’ incentive to commit to clearly delineated obligations.
Requiring a higher standard of proof when there is express contract language to the contrary comports with previous Michigan cases regarding modification. Generally, parties are free to take from, add to, or modify an existing contract. . . . However, in the same way a meeting of the minds is necessary to create a binding contract, so also is a meeting of the minds necessary to modify the contract after it has been made. ... A collective bargaining agreement, like-any other contract, is the product of informed understanding and mutual assent. To require a party to bargain anew before enforcing a right set forth in the contract requires proof that the parties knowingly, voluntarily, and mutually agreed to new obligations.
[572]*572. . . Once the employer has fulfilled its duty to bargain, it has a right to rely on the agreement as the statement of its obligations on any topic “covered by” the agreement. “[T]he courts require ‘clear and unmistakable’ evidence of waiver and have tended to construe waivers narrowly.”
In the present context, we agree with the merc that the fact that the charging party or other school districts may have tacitly or informally acquiesced to insurance benefit changes does not prevail over contradictory contract language under the standard of proof required by Port Huron, supra. An increase in the lifetime maximum benefit level from one million to two million dollars a person cannot be equated with providing wigs for those limited numbers of subscribers requiring this coverage. We affirm the commission’s decision that the school district did not waive its right to renegotiate modification of the term, nor did it extend to the MESSA the right to unilaterally make benefit changes to the insurance coverage specified in the contract.
v
The final argument advanced by the mea and the MESSA involves the assertion that, assuming arguendo the change in the benefit level was a midterm modification, the other terms of the collective bargaining agreement protected the employer because any increase in health care premiums during the life of the contract over and above the costs agreed upon were at “no cost” to the employer.
The contract is the manifestation of the give and take of bargaining regarding both economic and noneconomic issues. The monetary and nonmonetary value of those considerations are subsumed and inte[573]*573grated in the agreement. The assertion that a change in the contract terms would not impose a financial cost is no more a justification for an unassented modification of contradictory contract language than if the employer were to unilaterally extend the specified length of the work day.36
CONCLUSION
We affirm the merc’s determination. The school district did not waive its right to negotiate for the change and the commission properly required the union to reinstate the original level of benefits to the MESSA health care plan as ratified. Because the collective bargaining agreement had not terminated, any change or modification had to be mutually renegotiated. The charging party was entitled to insist on the terms that had been specifically bargained for and memorialized in the collective bargaining agreement. Respondent MEA violated subsection 10(3)(c) of the pera through the independent actions of the messa, its third-party agent.
Mallett, C.J., and Brickley, Weaver, and Taylor, JJ., concurred with Boyle, J.