Respondent, Southfield Public Schools, appeals as of right from a determination of the Michigan Employment Relations Commission (MERC) that the regular substitute custodial workers employed by the school system could appropriately be included in the same bargaining unit with the other educational support employees currently represented by petitioner, Michigan Educational Support Personnel Association (MESPA).
Respondent first asserts that MERC erred in its finding that the substitute custodians are entitled to bargaining rights. Respondent argues that since the substitute custodians’ right to work was entirely dependent upon the absences of the regular custodians, the substitute custodians were "casual” employees, and thus disqualified from membership in a unit of organized employees pursuant to the MERC decision in Jackson Public Schools, 1983 MERC Lab Op 579. Respondent also argues that whether substitute custodians are includable in the existing bargaining unit is a question of law, not fact, and thus respondent advocates a broad standard of review.
We disagree. Pursuant to MCL 423.9e; MSA 17.454(10.4). the task of determining an appropriate collective bargaining unit is left to MERC. Such a determination involves two inquiries, as noted by the Court in Food Store Employees Union, Local 347, Amalgamated Meat Cutters & Butcher Workmen of North America v National Labor Relations Board, 137 US App DC 248; 422 F2d 685, 689 (1969):
"The first question is which individuals are ’employ*717ees’ within the policy of the Act; the next is what group within the class of employees shares a community of interests such that representation by a single bargaining agent will meet the statutory goal of 'assuring] to employees the fullest freedom in exercising the rights guaranteed’ by the Act.” (Footnote omitted.)
Further, it is well settled that the determination of an appropriate unit is a finding of fact which will not be overturned by this Court if supported by competent, material, and substantial evidence. Michigan Coaches Ass’n v Warren, 119 Mich App 85, 88; 326 NW2d 432 (1982); Lansing School Dist v Michigan Employment Relations Comm, 117 Mich App 486, 489-490; 324 NW2d 62 (1982).
Respondent’s contention that the substitute custodians were mere "casual” employees and thus not entitled to membership in a unit of organized employees was rejected by MERC below. MERC began by noting that none of the cases cited by respondent supported the conclusion that "substitutes per se are not entitled to bargaining rights”, and thus an analysis of the particular facts presented was necessary. MERC first looked to the number of hours worked, and found that most substitutes averaged 30 or more hours per week and that eleven of the substitutes work approximately as many hours as the average for all full-time regular custodial employees. As to regularity of employment, although they could refuse work assignments, MERC found that this was not common and that many substitutes frequently worked five-day weeks.
Further, MERC noted that most substitutes were given "Letters of Assurance” at the end of the school year, indicating the respondent’s intention to rehire them the following school year. MERC considered this fact to be "evidence of the continuing nature and built-in expectations” that *718characterized both sides of the employment relationship. Regarding compensation, while substitutes were paid less than regular custodians, both served a probationary period at reduced wages, both received mileage reimbursement for on-the-job transportation, and both were eligible for in-service training.
Next, MERC considered the supervisory aspects of the relationship, and found that substitutes and regular employees shared the same on-site supervisors and reported to the same departmental supervisor. Identical criteria were used in the evaluation of the employees and both could be fired for unsatisfactory performance.
Our review of the record in this case leads us to conclude that MERC’s findings are supported by competent, material and substantial evidence. Unlike the substitute teachers in Lansing School Dist, 1978 MERC Lab Op 453, cited by respondent, the substitute custodians in the instant case maintained a strong employment relationship with respondent. We therefore affirm MERC’s finding that the substitute custodians were "employees” within the meaning of the public employment relations act (PERA), MCL 423.201 et seq.; MSA 17.455(1) et seq., rather than mere "casual” employees without the right to be included in an organized labor unit.
The next question to be addressed is whether the substitute custodians and the educational support employees already belonging to MESPA "share a community of interests such that representation by a single bargaining agent will meet the statutory goal of 'assuring to employees the fullest freedom in exercising the rights’ guaranteed by the Act”. Food Store Employees Union, supra, p 689. The objective in designating bargaining units is to create the largest possible unit consistent with a community of interest of the *719membership. Lansing School Dist v Michigan Employment Relations Comm, supra, p 490.
In this case, MERC concluded that the substitute custodians share a community of interest with the other regularly employed custodians sufficient to justify inclusion in the same bargaining unit. MERC based this conclusion on the fact that the two types of employees perform essentially the same work in the same buildings under the same supervision and that work assignments were delegated in the same manner for both regular and substitute custodians. MERC then noted that certain provisions contained in the present collective bargaining agreement could be considered antagonistic to the interests of the regular substitute custodians. For example, a concern among regular employees is the limitation of the number of hours made available to substitute custodians in order to prevent over-utilization of the lower paid substitutes. MERC nonetheless concluded that this was not enough to defeat the statutory right of the affected employees to organize together. The opinion states: "It is not uncommon for a labor organization to be required to balance the competing interests of various subgroups of employees, in defining specifically beneficial rights sought by its constituents.” We agree with this observation, and thus decline to reverse the commission’s findings and conclusions regarding the appropriate bargaining unit in which to include the substitute custodians. In doing so, we are not unmindful of the commission’s expertise in these matters, and thus we believe it is appropriate to give considerable deference to their analysis of issues such as those presented in this case.
We also reject respondent’s contention that the election held pursuant to a directive order of MERC, in which all 19 substitute custodians se*720lected MESPA as their collective bargaining agent, was barred by § 14 of PERA. Section 14 provides as follows:
"An election shall not be directed in any bargaining unit or any subdivision within which, in the preceding 12-month period, a valid election was held. The commission shall determine who is eligible to vote in the election and shall promulgate rules governing the election. In an election involving more than 2 choices, where none of the choices on the ballot receives a majority vote, a runoff election shall be conducted between the 2 choices receiving the 2 largest numbers of valid votes cast in the election. An election shall not be directed in any bargaining unit or subdivision thereof where there is in force and effect a valid collective bargaining agreement which was not prematurely extended and which is of fixed duration. A collective bargaining agreement shall not bar an election upon the petition of persons not parties thereto where more than 3 years have elapsed since the agreement’s execution or last timely renewal, whichever was later.” MCL 423.214; MSA 17.455(14).
In the instant case, the bargaining agreement between MESPA and the school district was to expire by its own terms on August 26, 1984. The direction of the election was made on March 2, 1984, within the effective period of the agreement. Accordingly, an election within the MESPA bargaining unit would appear to be barred by § 14. The substitute custodians, however, were not members of the bargaining unit. Since the prohibition on elections set forth in § 14 refers only to elections within the bargaining unit, such prohibition does not apply to the instant case.
Affirmed.