ALLEN, J.

Respondent Department of Treasury appeals as of right from a May 16, 1985, decision and order of the Michigan Tax Tribunal canceling a single business tax assessment of $21,991.08, plus interest, for the taxable years ending October 31, 1977, 1978 and 1979. The basis of the alleged deficiency was the taxpayer's failure to include in its tax base the compensation which the taxpayer paid to various "on site" employees. We affirm.

FACTS

Following receipt of the deficiency assessment, Mid America, on June 21, 1982, filed a petition for review by the Michigan Tax Tribunal. A hearing was held before hearing officer Michele Halloran on January 23, 1984, at which the following facts were presented. Petitioner is a corporate property manager involved primarily in the management of apartment complexes and shopping centers. Clients, hereinafter referred to as project owners, include individual property owners, syndicators, and developers in Michigan and Kentucky. Clients also include nonprofit organizations which are federally subsidized through the United States Department of Housing and Urban Development (HUD). Petitioner is one hundred percent owned by Joseph Schmitt, Jr., and his wife. Joseph Schmitt, Jr., has an ownership interest in several of petitioner's clients. The principal office is in the City of Flint. On-site employees, in most instances, live in the project owned by the project owner and generally include the resident manager and grounds and maintenance staff.

For tax purposes petitioner divided its employees into two groups: (1) office and management personnel and (2) on-site employees of the clients being served by petitioner. It was petitioner's position that on-site employees are the employees of the project owner; therefore, the compensation paid to the on-site employees is includable in the tax base of the project owner. Mid America Management offered as evidence in support of its position (1) the management agreement; (2) the testimony of Mr. Schmitt (President of Mid America Management) and Mr. Dietz (a project owner) to the effect that the owner retains the right to hire, fire and supervise the on-site employees, and that on occasion this right is exercised; and (3) the corporate treasurer's testimony, which reveals that the project owner reimbursed the management company for the payroll.

The department, on the other hand, took the position that Mid America is the employer of the on-site employees because Mid America controls the day-to-day operation of the business, i.e., the hiring, firing and supervising of all employees, and it controls the payroll for all employees. Therefore, according to the department, the entire compensation paid should be includable in the tax base of Mid America.

Another fact presented at the hearing and pertinent to the issue before us is that one of the project owners, Paul Dietz, testified that the single business tax returns for his project were always filed with on-site employees being treated as employees of the project. No other proof on this point was presented at the hearing.

Other witnesses testified that clients engaged petitioner's services by entering into one of two standard management agreements. One agreement was used for federally subsidized projects and contained detailed terms which are required and approved by HUD. The standard management agreement for nonsubsidized projects contained more general language, but under both agreements petitioners had authority to hire, discharge, and supervise on-site employees. Mid America was also to perform all payroll functions, subject to the project owner's obligation to reimburse petitioner for all salaries, wages, related payroll taxes and fringe benefits. Mid America's fee for its management services is computed at four percent to six percent of gross collections, which includes rental and other miscellaneous income of the project.

Petitioner's primary duties as interpreted by its president, Joseph Schmitt, Jr., include (1) maximizing a project's income, (2) operating the project within the budget approved by the project owner, (3) hiring and training the resident manager of the project, and (4) insuring that the project is physically maintained, cosmetically appealing and, consequently, marketable. Testimony concerning the respective powers and duties of petitioner and project owners vis-a-vis on-site employees is summarized below.

Management Agreement Provisions: The management agreement for federally subsidized projects provides that (1) all on-site employees will be employees of the project owner and not petitioner, (2) on-site employees will be hired, paid, discharged, and supervised through petitioner, acting as the project owner's agent, (3) all compensation paid to on-site managerial and maintenance employees will be reimbursed to petitioner, as agent of the project owner, (4) compensation for the on-site manager and maintenance employees will be as prescribed in the management plan, and (5) the compensation for bookkeeping, clerical, and other managerial personnel will be within the sole discretion of petitioner. Thus, the management agreement for subsidized projects clearly delineates the responsibilities of petitioner in relation to on-site employees. The management agreement for non-subsidized projects contains similar responsibilities, but is in substantially less detail.

Procedures for Hiring and Firing On-Site Employees: Petitioner has its own "property management department." Each property manager in that department is responsible for seven or eight projects. The property manager hires and trains the on-site resident manager. The project owner also has authority to hire, fire and supervise the resident manager and in some cases has actually chosen to do so. Paul Dietz, for instance, stated that he actively participated in hiring and termination decisions in his capacity as a project owner. If a conflict over which decision to make occurred, Dietz's decision controlled. Petitioner would then act as Dietz's agent and hire and fire as he instructed.

All other on-site employees are hired by the resident manager. Petitioner's written management and employment policies, including its fringe benefit and pay scales, are brought into the project. Some economy-of-scale benefits are obtained by this arrangement. In particular, insurance policies are centrally obtained by petitioner and then reimbursed by the project owners at the lower premium rates. If a project owner objects to petitioner's policies, then the project owner's decision controls the matter. Basically, petitioner tailors its management plan to meet the project owner's objective.

Pertinent Financial Procedures: A separate banking account is maintained by petitioner for each project. The resident manager deposits rental and other miscellaneous receipts from the project into the operating account. A single payroll clearing account titled "Mid America Management Payroll Account" is maintained by petitioner for all payroll and related expenses, including the payroll for petitioner's corporate employees and the on-site employees of each project. For payroll purposes, petitioner's corporate employees are essentially treated as employees of a corporate "project."

Each employee prepares a time sheet, which is then signed by the employee and the employee's supervisor. In the case of on-site employees, the resident manager usually approves the time sheet. Time sheets are submitted in batches, by project, to payroll clerks at petitioner's administrative office. Payroll clerks then summarize the information from the time sheets onto a work sheet by expense class code. The information is then given to an independent computer service which produces a payroll report by project and prints the employees' paychecks. All checks are drawn against the single clearing account covering all projects mentioned above.

The paychecks and payroll are then submitted to petitioner and signed by petitioner's treasurer. Upon their receipt at the various projects, the project resident manager distributes the checks to the on-site employees. At the same time, petitioner also prepares separate checks drawn on each project's bank account for the total amount of checks issued for each project. The checks so drawn on each project's operating account are signed by petitioner's treasurer and deposited in the single clearing account. The net effect is to reimburse the clearing account for the total sums paid to each project's employees. Thus, all payroll and related expenses are paid from the project's separate bank account. However, the reimbursement procedure also results in each project's payment of salaries and wages being reported to the federal government on federal forms 940 and 941 as compensation paid by Mid America rather than by the project.

On February 15, 1985, the hearing officer issued a proposed judgment affirming the assessment in its entirety. The proposed judgment found that although the project owner was the common-law employer of on-site employees, Mid America was the statutory employer, since it possessed control over the payment of wages to the on-site employees.

In the instant case, despite the fact that the individual project owners fall within the ambit of the federal regulation's definition of common law employer, Mid America nevertheless must be deemed the statutory employer of the on-site workers under the prevailing exception stated in 26 U.S.C. § 3401(d)(1) (and consequently under the applicable state definition as well), for the reason that it possessed sole control of the payment of these persons' wages. As the employer of the on-site personnel, the taxpayer thus was required to include compensation paid to these persons — its employees — within its tax base under sec 9(5) of the single business tax act.

Mid America filed exceptions and written objections with the Tax Tribunal. The tribunal, upon review of the proposed judgment and the record, reversed the decision of the hearing officer and cancelled the assessment. The tribunal held that the employment relationship should be determined by the traditional common-law tests rather than under 26 U.S.C. § 3401(d)(1).

We also agree with the conclusion in the Proposed Judgment that the project owners retained control and direction over the on-site employees and that these employees, therefore, were the common law employees of the project owner. However, we disagree with the Hearing Officer's conclusion that the definition of "employee" in 26 U.S.C. § 3401(c) was of little assistance to adjudge Petitioner's liability. Contrary to the assertions by the Department of Treasury during this case, the policy stated by the Department of Treasury in its own "Michigan Single Business Tax Questions Answers", Volume I-XIII and Bulletins, C-8015 (Rev. 5/82), Bulletin 1981-1, "Compensation", the employment relationship is to be ascertained with reference to the traditional common law tests, and not according to the more technical rules established in 26 U.S.C. § 3401(d):

* * *

In conclusion, since the purpose of the Act is to impose a tax upon economic activity in the state ( Kelvinator v Department of Treasury, 136 Mich. App. 218 [ 355 N.W.2d 889], 1984), the entity that is responsible for the tax should be the entity that benefitted from such economic activity. The value added to the economy can only originate from the true common law employer. The project owner was the true beneficiary of the efforts of the on-site employees. Any value added to the economy as a result of the efforts of these employees must be taxed to the owners under the Single Business Tax Act.

ISSUES

On appeal the department contends that the tribunal erred in two controlling respects: (1) by giving considerations to parol evidence which contradicted the clear and unambiguous language of the management agreement, and (2) by impermissibly excluding from the single business tax base a significant and essential part of the business activity, viz.: the compensation paid to a select number of employees engaged in the business which is being taxed. In rebuttal, Mid America argues (1) that the language of the management agreement was not so clear and unambiguous as to make parol evidence inadmissible, and (2) under criteria set forth in Treas Reg 31.3401(c)-1(b), on-site employees are the common-law employees of the project owners, not of Mid America, and as such are expressly excluded from Mid America's tax base.

DISPOSITION

I

On the threshold issue of whether the on-site employees were the employees of Mid America or of the project owners, the hearing officer found that the project owners were the common-law employers because the credible and unrebutted testimony of petitioner's president, Joseph Schmitt, Jr., and project owner, Paul Dietz, showed that, while the management agreements did not per se reserve to the project owners the right to hire, discharge, and supervise on-site employees, in actual practice the right to direct and control was retained by the project owners and at times was exercised. The hearing officer relied on the general common-law principles embodied in the federal treasury regulation to determine whether a common-law employer-employee relationship existed. In 1981, the department adopted federal regulation 26 C.F.R. § 31.3401(c)-1 and (d)-1 for determining whether an employer-employee relationship existed. That regulation provides:

(b) Generally the relationship of employer and employee exists when the person for whom services are performed has the right to control and direct the individual who performs the services, not only as to the result to be accomplished by the work but also as to the details and means by which that result is accomplished. That is, an employee is subject to the will and control of the employer not only as to what shall be done but how it shall be done. In this connection, it is not necessary that the employer actually direct or control the manner in which the services are performed; it is sufficient if he has the right to do so. The right to discharge is also an important factor indicating that the person possessing that right is an employer. Other factors characteristic of an employer, but not necessarily present in every case, are the furnishing of tools and the furnishing of a place to work to the individual who performs the services. In general, if an individual is subject to the control or direction of another merely as to the result to be accomplished by the work and not as to the means and methods for accomplishing the result, he is not an employee. [26 C.F.R. § 31.3401(c)-1.]

This common-law test primarily focuses on the right to direct and control the manner and method of providing a service, although other factors may be considered. Marvel v United States, 719 F.2d 1507 (CA 10, 1983). The department argues that under the parol evidence rule the hearing officer was precluded from giving consideration to the testimony of Schmitt and Dietz. Under the parol evidence rule, when two parties have reduced to writing that which they agree is a complete and accurate statement of their agreement, extrinsic evidence of antecedent and contemporaneous understandings is inadmissible for the purpose of varying or contradicting the writing. Van Pembrook v Zero Mfg Co, 146 Mich. App. 87, 97-98; 380 N.W.2d 60 (1985).

No objection was raised by the department to the testimony of Schmitt and Dietz and, consequently, petitioner contends the issue is not preserved. Ordinarily, an objection must be raised to preserve appellate review of evidentiary questions. City of Warren v Maccabees Mutual Life Ins Co, 83 Mich. App. 310, 319; 268 N.W.2d 390 (1978), lv den 405 Mich. 845 (1979). Because the parol evidence rule is a rule of substantive law as well as a rule of evidence, Salzman v Maldaver, 315 Mich. 403, 412; 24 N.W.2d 161 (1946), its applicability may be considered even if the parol testimony is admitted in evidence without objection. Accordingly, we address the issue on its merits.

The sine qua non of the parol evidence rule is a finding that the parties intended the writing to be a complete expression of their agreement as to the matters covered. Extrinsic evidence is admissible to resolve this threshold question. NAG Enterprises, Inc v All State Industries, Inc, 407 Mich. 407; 285 N.W.2d 770 (1979). Furthermore, the rule does not preclude admitting extrinsic evidence to resolve an ambiguity which is proven to exist and, thereafter, to determine the actual intent of the parties. Goodwin, Inc v Orson E Coe Pontiac, Inc, 392 Mich. 195; 220 N.W.2d 664 (1974). This is consistent with the rule that contract construction is ordinarily a question of law for the court. When the language is ambiguous or incomplete, or circumstances unusual, then the substance of the parties' agreement is a factual question. Zinchook v Turkewycz, 128 Mich. App. 513; 340 N.W.2d 844 (1983).

Admittedly paragraph 3(c) of the management agreement clearly and unambiguously gives Mid America the authority to hire, fire, and supervise on-site employees at the various complexes. The problem is, the management agreement does not, either expressly or by implication, confer exclusive authority on management to hire, discharge and supervise. Left in the air is whether project owners had the same power and, more particularly, if a conflict arose, who prevailed. Therefore, parol evidence was admissible to determine the threshold issue of whether the parties intended the written agreement to embody the complete agreement on the matter of hiring, firing and supervision of on-site employees. The hearing officer found the testimony of Schmitt and Dietz to be credible. Indeed, their testimony that the project owners had ultimate control in the event of a conflict between Mid America and the project owner was unrebutted.

This Court's review of Tax Tribunal decisions is limited to determining whether they are authorized by law and whether the factual findings are supported by competent, material and substantial evidence on the whole record. MCI Telecommunications Corp v Dep't of Treasury, 136 Mich. App. 28; 355 N.W.2d 627 (1984), lv den 422 Mich. 883 (1985); Const 1963, art 6, § 28. "Substantial evidence" must be more than a scintilla of evidence, although it may be substantially less than the preponderance of evidence necessary in most civil cases. Holy Spirit Ass'n for the Unification of World Christianity v Dep't of Treasury, 131 Mich. App. 743; 347 N.W.2d 707 (1984). In view of the fact that the testimony was found credible by the hearing officer and that no testimony in rebuttal was presented, we also hold that the hearing officer's finding that the project owner was the common-law employer is supported by competent, material and substantial evidence.

II

Our determination that petitioner was not the common-law employer of the project owner's on-site employees does not in itself mean that the deficiency assessment of $28,068.64 was properly canceled. Still to be decided is the main issue of whether the compensation paid to the on-site employees by petitioner and in turn reimbursed by the project owners was properly included in petitioner's adjusted tax base for purposes of the Single Business Tax Act (SBTA). Resolution of this issue first requires an explanation of the basic principles of the SBTA.

The SBTA, MCL 208.1 et seq.; MSA 7.558(1) et seq., imposes a specific tax on the adjusted tax base of every person with business income in this state. MCL 208.31(1); MSA 7.558(31)(1). It is best understood as a value added tax, although it is not a pure value added tax. Town Country Dodge, Inc v Dep't of Treasury, 420 Mich. 226, 234; 362 N.W.2d 618 (1984). "Adjusted tax base" means business income, subject to certain adjustments as described in MCL 208.9; MSA 7.558(9). Business income is defined as federal taxable income. MCL 208.3(3); MSA 7.558(3)(3). One of the adjustments to the tax base requires the taxpayer to add to taxable income the compensation paid its employees. MCL 208.9(5); MSA 7.558(9)(5); Jones Laughlin Steel Corp v Dep't of Treasury, 145 Mich. App. 405, 407; 377 N.W.2d 397 (1985), lv den 424 Mich. 895 (1986). "Compensation" means all wages, salaries, fees, bonuses, commissions or other payments made in a taxable year on behalf of or for the benefit of employees. Compensation includes certain fringe benefits and employment taxes. MCL 208.4(3); MSA 7.558(4)(3).

The terms "employee" and "employer" are defined as stated in the Internal Revenue Code, 26 U.S.C. § 3401(c) and (d), respectively. MCL 208.5; MSA 7.558(5). 26 U.S.C. § 3401 et seq. establishes rules for withholding federal income tax at the source of "wages." An individual is an employee for federal employment tax purposes if he has the status of an employee under the usual common-law rules applicable in determining the employee-employer relationship. Employment tax regulations establish guidelines for determining that status. Marvel v United States, 719 F.2d 1507 (CA 10, 1983); 26 C.F.R. § 31.3401(c)-1 and 31.3401(d)-1. A person from whom an employer is required to withhold for federal income tax purposes and a person required to withhold for federal income tax purposes shall prima facie be deemed an "employee" and "employer" respectively. MCL 208.5; MSA 7.558(5).

Under the above definitions the project owner was clearly the common-law employer of the on-site employees. Nevertheless, the department contends that under the emphasized language set forth below in the exception found in 26 U.S.C. § 3401(d)(1), petitioner must be deemed the employer because it controls the payroll. 26 U.S.C. § 3401(d)(1) provides:

(d) Employer. For purposes of this chapter, the term "employer" means the person for whom an individual performs or performed any service, of whatever nature, as the employee of such person, except that —

(1) if the person for whom the individual performs or performed the services does not have control of the payment of the wages for such services, the term "employer" (except for purposes of subsection [a]) means the person having control of the payment of such wages. . . . [Emphasis added.]

We disagree on three grounds. First, to fall within the exception, control must be exclusive. Nosanchuk v Dep't of Treasury, 126 Mich. App. 366, 368; 337 N.W.2d 66 (1983). As heretofore noted in issue I, petitioner's control over wages was far from exclusive.

Second, as noted earlier in issue I and also noted in the tribunal's written opinion, the department itself published a policy statement that it is the legal relationship of an employer and employee which is relevant to questions arising under the SBTA, and not the technical regulations found in the federal income tax act.

Third, we do not believe that the Legislature intended that the statutory exception contained in 26 U.S.C. § 3401(d)(1) should determine who will be treated as an employer for purposes of the SBTA. Section 5 of the SBTA makes its own reference to § 3401(d) and we feel that reference rather than the more specific reference made by the hearing officer applies. Section 5 of the SBTA reads in pertinent part:

Sec. 5(1). "Employee" means an employee as defined in section 3401(c) of the internal revenue code. A person from whom an employer is required to withhold for federal income tax purposes shall prima facie be deemed an employee.

(2) "Employer" means an employer as defined in section 3401(d) of the internal revenue code. A person required to withhold for federal income tax purposes shall prima facie be deemed an employer. [Emphasis added. MCL 208.5; MSA 7.558(5).]

In construing a statute which is unambiguous on its face, interpretation and construction of its terms are unnecessary. If an ambiguity exists, then this Court must give effect to legislative intent. Sneath v Popiolek, 135 Mich. App. 17; 352 N.W.2d 331 (1984). To resolve a perceived ambiguity, this Court looks to the object of the statute, and will apply a reasonable construction which best accomplishes the statute's purpose. In the Matter of Colon, 144 Mich. App. 805, 810; 377 N.W.2d 321 (1985).

On its face, it appears that the exception contained in § 3401(d)(1) should apply to the SBTA because § 5 expressly looks to 26 U.S.C. § 3401(d) for the meaning of an employer. An ambiguity occurs, however, when one attempts to reconcile the intended purpose of 26 U.S.C. § 3401(d)(1), which is to fix responsibility for withholding employment taxes, to the purpose of the SBTA, which is intended to tax the incremental value of a taxpayer's economic activity. Kelvinator, Inc v Dep't of Treasury, 136 Mich. App. 218; 355 N.W.2d 889 (1984), lv den 421 Mich. 861 (1985). The technical exception contained in 26 U.S.C. § 3401(d)(1), although appealing from an administrative standpoint, has no application to the purpose of the SBTA.

Having determined that the controlling statutory provision is § 5(1) and (2) of the SBTA, we next address the meaning of the "prima facie" language contained therein. In its brief the department argues that when a person withholds for federal tax purposes from the wages of another who has performed a service, an irrebuttable presumption is created that the person is the employer. Mid America argues that this only creates a rebuttable presumption and also requires that the person have legal control over the payment of wages. Which position is correct?

A conclusive presumption in a statute cannot be rebutted by testimony. Pearo v City of Mackinac Island, 307 Mich. 290; 11 N.W.2d 893 (1943). By contrast, it is well settled that a rebuttable presumption makes out a prima facie case and casts the burden of proof on the opposing party. Krisher v Duff, 331 Mich. 699; 50 N.W.2d 332 (1951). Here, the Legislature did not expressly provide for a conclusive presumption. Upon reviewing the statute as a whole, we opine that the Legislature intended to create a rebuttable presumption and, thereby, to put the burden of proof on the opposing party.

In the case at bar, the rebuttable presumption (assuming Mid America was required to withhold for federal income tax purposes), was that Mid America was the employer of the on-site employees. Petitioner satisfactorily rebutted that presumption through the testimony of its president, Joseph Schmitt, Jr., and a project owner, Paul Dietz.

Finally, we reject the argument that the management agreement in the instant case was executed with the purpose of improperly shifting the tax burden from petitioner to the project owners. That assertion is not supported by the facts. Instead, the reimbursement provisions appear to be the normal method by which to conduct business between the parties under the facts in this case. In summary, having found the tribunal's decision was authorized by law and is supported by competent, material and substantial evidence, the decision is affirmed.

Affirmed. No costs, a question of public importance being involved.