(dissenting). I respectfully dissent. I would reverse the ruling of the Michigan Employment Relations Commission (MERC) that respondent, Detroit Board of Education, did not commit any unfair labor practices and remand to the MERC for it to fashion an appropriate remedy in light of a finding that respondent committed unfair labor practices.
i
I concur with the facts as set forth by the majority and reiterate the standard of review. Factual findings of the MERC are conclusive if supported by competent, material, and substantial evidence on the whole record. MCL 423.216(e); MSA 17.455(16)(e); Const 1963, art 6, § 28; Amalgamated Transit Union, Local 1564, AFL-CIO v Southeastern Michigan Transporta*71tion Authority, 437 Mich 441, 450; 473 NW2d 249 (1991). Review of factual findings of the MERC is to be undertaken with sensitivity, and due deference is to be accorded to its administrative expertise. Id. 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. Id.
Legal rulings of administrative agencies are not given the deference accorded factual findings. Legal rulings of an administrative agency are set aside if they are in violation of the constitution or a statute, or affected by a substantial and material error of law. MCL 24.306(1)(a) and (f); MSA 3.560(206)(1)(a) and CO; Amalgamated Transit Union, supra, p 450.
n
The first argument of the charging party, Organization of School Administrators and Supervisors, AFSA, AFL-CIO (osas), is that the merc erred in finding that respondent did not violate its statutory duty to bargain when it unilaterally reduced rates of pay and hours of work for certain school administrators. The MERC found that respondent was not required to bargain over the hours of work for the school administrators because in the collective bargaining agreement the union had waived its right to demand bargaining over changes in the hours of jobs. With respect to rates of pay, the MERC determined that a bona fide dispute existed regarding whether the collective bargaining agreement was breached and that this dispute would have to be resolved through the parties’ grievance arbitration mechanism. However, in the order following the OSAS’s motion for reconsideration, the *72MERC stated that it found that respondent committed no unfair labor practice with respect to changing the method of compensation.
Under subsection 15(1) of the public employment relations act (pera), public employers have a duty to bargain over wages, hours, and other terms and conditions of employment. MCL 423.215(1); MSA 17.455(15)(1). Under the pera, an employer commits an unfair labor practice if, before bargaining, it unilaterally alters or modifies a term or condition of employment, unless the employer has fulfilled its statutory obligation or has been freed from it. Port Huron Ed Ass’n v Port Huron Area School Dist, 452 Mich 309, 317; 550 NW2d 228 (1996). An employer may defend against a charge that it has unilaterally altered working conditions by arguing that the union has waived its right to demand bargaining. Id., p 318.
A waiver occurs when a union knowingly and voluntarily relinquishes its right to bargain about a matter. Dep’t of Navy v Federal Labor Relations Authority, 295 US App DC 239, 248; 962 F2d 48 (1992). 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 an employer, and instead gives full discretion to the employer regarding the matter. Id. The courts require clear and unmistakable evidence to support a claim that the union has waived the statutory right. Id.; Port Huron, supra, p 320.1
*73Section 9.0 of the collective bargaining agreement contains the following:
9.0 HOURS OF WORK
In order to attain ultimate efficiency in the operation of the Detroit Public Schools and to provide the best possible educational program to the pupils served thereby, it is essential for administrative and supervisory personnel to work a schedule which reasonably permits the flexibility necessary for the achievement of such goals. Such a schedule may, at times, involve work in and out of the school building and, at times, frequently outside the regular school day. The professional discretion of good administrators and supervisors in scheduling their hours of work shall be respected insofar as such discretion is reasonable and is consistent with the school program and the aforestated aims. Professional discretion may be denied for good and sufficient reason. It is understood that this provision does not preclude the setting of hours by the Board or the General Superintendent when necessary, and is not intended to supersede the requirements of any leave policy. In instances where the General Superintendent or Administrative Heads have established regular work hours, prior approval for any deviation by administrators and supervisors is required. [Emphasis added.]
The MERC found that this section (particularly the emphasized portion) in the collective bargaining agreement established that the osas waived its right to demand bargaining over changes in the hours of jobs within the bargaining unit.
I would find that the MERC erred as a matter of law in interpreting § 9.0 to mean that the osas waived its right to demand bargaining over changes in the hours of jobs. The initial question whether contract lan*74guage is ambiguous is a question of law. Port Huron, supra, p 323. If the contract language is clear and unambiguous, its meaning is a question of law. Id. Where the contract language is unclear or susceptible to multiple meanings, interpretation is a question of fact. Id. Although I agree with the MERC that the contractual provision is unambiguous, I disagree with its interpretation of the provision.
In reading the provision as a whole, there is no waiver by the union of its right to demand bargaining concerning unilateral changes by the employer regarding hours of work. Section 9.0 deals with the scheduling of hours for administrative and supervisory personnel and permits flexible schedules. Section 9.0 clearly permits administrators and supervisors the discretion to schedule their work hours, but provides that such exercise of discretion must be consistent with the school program and be reasonable. The sentence seized upon by the MERC, when properly understood, merely allows respondent to set schedules for administrators and supervisors when necessary. In other words, administrators and supervisors do not always work normal “nine to five” business hours. Therefore, administrators and supervisors are accorded discretion in setting their workday hours under § 9.0. However, if such discretion is abused by an administrator or supervisor, then respondent may set work hours. Thus, respondent could demand that a supervisor must begin work at 8:30 A.M., rather than 9:00 A.M., but the language says nothing about reducing the number of hours worked. Accordingly, § 9.0 permits respondent to set a fixed workday or schedule for the administrators and *75supervisors, but it does not waive the union’s right to demand bargaining over changes in the hours of jobs.
The difference here is significant. Section 9.0 allows respondent to set schedules; it does not waive the osas’s right to bargain over the unilateral reduction in hours. There is no “clear and unmistakable waiver” by the union with respect to the employer’s unilateral decision to reduce the number of hours worked. See, e.g., Wayne Co Government Bar Ass’n v Wayne Co, 169 Mich App 480, 487; 426 NW2d 750 (1988) (the union did not clearly and unmistakably waive its right to bargain over cost-of-living-allowance benefits after the expiration of the collective bargaining agreement where the contractual language stated that the payments for the benefits would “be made quarterly during the term” of the collective bargaining agreement); Kent Co Ed Ass’n v Cedar Springs Public Schools, 157 Mich App 59, 66; 403 NW2d 494 (1987) (a “zipper” clause in a collective bargaining agreement waiving the right to collective bargaining on matters covered by the agreement was not a clear and explicit waiver of the right to bargain over the change in the number of high school class periods absent any specific reference in the agreement to the number of class periods). Therefore, I would conclude that the merc’s decision was affected by a substantial and material error of law when it did not properly interpret the parties’ collective bargaining agreement. Because hours constitute a mandatory subject of bargaining, respondent could not unilaterally change the number of hours without first bargaining over that condition, absent impasse or waiver. Ottawa Co v Jaklinski, 423 Mich 1, 13; 377 NW2d 668 (1985); Port Huron, supra, p 317. Therefore, under *76the PERA, respondent has committed an unfair labor practice in unilaterally changing the hours to be worked without first bargaining with the osas.
in
Respondent, in its cross appeal, argues that the merc’s decision regarding hours of work should be affirmed by this Court for a different reason. Respondent maintains that the hearing referee was correct in ruling that because the hours at issue constituted overtime, this was not a mandatory subject of bargaining and respondent could unilaterally reduce such hours. The MERC rejected the hearing referee’s reasoning in this regard and ruled that the evening work assignments “clearly fell within the scope of the work performed by the bargaining unit and covered by the contract.”
First, I agree with the majority that respondent has standing to raise this issue. Pursuant to MCL 423.216(e); MSA 17.455(16)(e), any party aggrieved by a final order may seek review of the merc’s order in this Court with respect to an unfair labor practice charge. The MERC specifically ruled that the work assignments at issue were not overtime, but were bargaining unit work. Such a ruling directly affects respondent’s bargaining rights because overtime is not part of regular wages and may be unilaterally reduced by an employer. Roseville v Police Officers Ass’n of Michigan, 1987 MERC Lab Op 182; St Clair Co Rd Comm v Local 516M, Service Employees Int’l Union, AFL-CIO, 1992 MERC Lab Op 316. Therefore, respondent is an aggrieved party because the merc’s order affects respondent’s rights and bears directly on its interests. Grace Petroleum Corp v Public Service *77Comm, 178 Mich App 309, 312-313; 443 NW2d 790 (1989); Midland Cogeneration Venture Ltd Partnership v Public Service Comm, 199 Mich App 286, 304; 501 NW2d 573 (1993).
Further, I agree that the merc’s factual finding that the assignments in the adult education program and vocational education program were within the scope of the work performed by the bargaining unit and covered by the contract is supported by competent, material, and substantial evidence on the whole record.2 The merc’s factual findings in this regard are:
The Administrative Law Judge concluded that Respondent had no duty to bargain over the elimination of overtime/supplemental assignments in the adult education program because these assignments were not bargaining unit work and had no impact on bargaining unit work. First, we disagree with the Administrative Law Judge that these assignments fell into the category of “overtime.” Both the adult basic education administrators and the vocational education administrators had two separate jobs. First, they administered Respondent’s daytime adult education or vocational education programs, for which they were paid a salary. Secondly, they administered Respondent’s evening adult education programs. The adult basic education administrators were paid an hourly rate for their second job. The *78vocational education administrators were paid on a per diem basis.
We also disagree with the Administrative Law Judge that' the administrators’ evening jobs did not constitute bargaining unit work. This work clearly fell within the scope of the work performed by the bargaining unit and covered by the contract. Moreover, it was treated as such by the parties, as exemplified by the fact that Respondent gave Charging Party notice of its proposed actions. The fact that the evening programs were administered by the same administrators who supervised the daytime programs is not relevant to the question of whether the work constituted bargaining unit work.
The merc’s factual findings in this regard are conclusive because they are supported by competent, material, and substantial evidence on the whole record. MCL 423.216(e); MSA 17.455(16)(e). The merc’s finding that the adult basic education administrators and the vocational education administrators had two separate jobs is supported by the- testimony of Dr. Kimberly Peoples, the director/administrator of the Golightly Career and Technical Center. She testified that she was paid a daily rate for her work in the evening program before February 1994, and that after the February 1994 change, she was paid an hourly rate for her evening work. Further, she testified that her “regular day school job” was from 7:30 AM. until 3:15 P.M., and that her adult education work was from 3:15 p.m. until 10:00 P.M. Dr. Peoples also testified that she was a salaried employee. Further, Dr. Arthur Carter, respondent’s deputy superintendent of government relations and community services, testified that the full-time job of vocational technical director was separate and distinct from the evening job of department head of a vocational technical center.
*79The merc’s finding that the administrators’ evening jobs constituted bargaining unit work and was covered by the contract is also supported by competent, material, and substantial evidence on the whole record. Sections 8.5 (evening school) and 8.6 (adult education) of the collective bargaining agreement specifically provide for these work assignments.
Accordingly, the merc’s factual finding that the evening hours at issue did not constitute overtime hours is conclusive because that finding is supported by competent, material, and substantial evidence on the whole record.
IV
As its final issue, the osas argues that the merc’s deferral to arbitration of the issue regarding respondent’s unilateral change in the method of pay for vocational education administrators was a substantial and material error of law.
The merc found that there was no unfair labor practice with regard to changing the method of pay and that the dispute concerned whether the contract was breached by the change in the method of compensation (from a daily to an hourly basis).3 I believe that the merc’s decision in this regard was affected by a *80substantial and material error of law. MCL 24.306(1)(a) and (f); MSA 3.560(206)(1)(a) and (f).
A particular course of conduct may violate contractual and statutory duties. Port Huron, supra, p 320. In this case, the osas contends that respondent had a duty to bargain over the change of the method of compensation because it is a mandatory subject of bargaining. Clearly, changing the method of compensation from daily to hourly affected wages. See, e.g., Jackson Community College Classified & Technical Ass’n, MESPA v Jackson Community College, 187 Mich App 708, 713; 468 NW2d 61 (1991). Wages are a mandatory subject of bargaining. Id.; Ottawa Co, supra, p 13. Under the pera, an employer commits an unfair labor practice if, before bargaining, it unilaterally alters or modifies a term or condition of employment, unless the employer has fulfilled its statutory obligation or has been freed from it. Port Huron, supra, p 317.
In its decision and order regarding the motion for reconsideration, dated April 22, 1996, the MERC specifically stated that it found that no unfair labor practice was committed under these circumstances. I must disagree with this ruling because respondent unilaterally changed a method of compensation for the evening classes. Because this affects wages, it was a mandatory subject of bargaining and could not be unilaterally altered by the employer. Therefore, respondent committed an unfair labor practice under the pera when it unilaterally altered the method of compensation. Respondent had a statutory duty to *81bargain with the osas concerning any change in the method of compensation because it directly affected wages.
Accordingly, I would reverse the MERC’s ruling that respondent did not commit any unfair labor practices when it unilaterally changed hours of work and the method of compensation. They are mandatory subjects of bargaining and there was no waiver by the union or a fulfillment of respondent’s statutory duty to bargain. I would remand to the MERC for it to fashion an appropriate remedy in light of this finding.
In Dep’t of Navy, supra, p 247, the court explained that a union will usually clearly and unmistakably waive its right to bargain either by an express agreement (for example, a “zipper clause”) or through its bargaining history with the employer. A zipper clause is a provision in a collective bargaining agreement that purports to close out bargaining during the *73conixact term and to make the written contract the exclusive statement of the parties’ rights and obligations. Id., p 247, n 8.
At this point, I take issue with the majority’s decision to vacate the merc’s finding that the adult education department heads’ evening work was. not overtime on the basis that the osas’s counsel conceded during oral argument that the department heads’ work was overtime. During oral argument, the osas’s counsel explained that the union did not challenge the employer’s right to say that the department heads could not work in the evening without permission, but objected to the employer’s unilateral creation of the compensatory time schedule. I do not take this statement to be a concession that the department heads’ evening work was overtime, not subject to mandatory bargaining. In any event, I would agree with the union that respondent was under a statutory duty to bargain regarding the creation of the compensatory time schedule.
I note that the merc’s rulings in this regard appear to be contradictory. In the February 2, 1996, decision and order, the merc stated that the dispute regarding pay was “a bona fide dispute over whether this provision of file contract was breached by the change in the method of compensation.” The merc then stated that the dispute should be resolved through the parties’ grievance arbitration mechanism. In its April 22, 1996, decision and order, the merc stated with respect to the method of pay issue, “we simply found that no unfair labor practice was committed under these circumstances.” In fact, the merc did not so find in the previous decision. However, the merc’s finding that there was no unfair labor practice committed with respect to the change in the method of pay should be addressed because this constitutes a statutory unfair labor practice issue *80that cannot be deferred to private arbitration. See Detroit Fire Fighters Ass’n v Detroit, 408 Mich 663; 293 NW2d 278 (1980).