OPALA, Chief Justice.

The issues to be resolved in this appeal are: (1) Does American Association of Retired Persons (AARP) have standing to seek corrective relief from the order tendered for review? (2) Is a public service corporation, i.e., Southwestern Bell Telephone Company (SWB), required by law to refund to its ratepayers surplus cash generated by a tax law change? (3) In determining the sum of surplus cash accumulated by SWB, may the Corporation Commission (Commission) include accrued interest? (4) Was it error for the Commission to direct SWB to apply the surplus cash toward upgrading the multi-party line areas of the state to single-party line service? (5) Is the decision to upgrade central offices supported by substantial evidence? (6) Did the Commission impermissibly enlarge its inquiry into the effects of federal tax law changes upon Oklahoma public utilities by also considering evidence of changes in SWB's business operations? (7) If not, (a) Is the Commission's finding that a depreciation reserve deficiency exists supported by substantial evidence? (b) Does SWB's failure to produce certain audit-related information afford the Commission legally sufficient grounds to exclude severance pay expense from SWB's revenue requirements? (c) In calculating SWB's "cash working capital" was it error for the Commission to exclude "local service revenues billed in advance?" and (d) Must the Commission have attributed interest to SWB's excess deferred income taxes? We answer questions 1, 3 and 7(c) in the affirmative and questions 2, 4, 5, 6 and 7(a), (b) and (d) in the negative.

I.

THE CRITICAL FACTS IN LITIGATION

In October of 1986 the Director of the Corporation Commission's Public Utility Division, Howard W. Motley, Jr., initiated proceedings to "inquir[e] into the effect of the 1986 Tax Reform Act on Oklahoma utilities." Twelve companies, including Southwestern Bell Telephone Company (SWB), were named as respondents and ordered to participate in a "technical conference" with the Commission's staff (Staff) for the purpose of "establish[ing] the scope of the information required to determine the effect of the Tax Act." According to the appellate record, the proceeding's next stage began on June 12, 1987, when notice was given to SWB and to "all interested parties" of a "hearing on the rates of Southwestern Bell Telephone Company" to be held on June 23, 1987. On the latter date, Staff and SWB stipulated in writing that if the Commission, after taking into account "all known and measurable changes" in SWB's business, determines a reduction in the utility's rates to be warranted, the reduced rates would become effective July 1, 1987, the effective date of the Tax Reform Act of 1986. The Commission approved the stipulation.

At the June 23, 1987 hearing the accounting manager for the Commission's Public Utility Division testified that "the 1986 Tax Reform Act reduced the corporate income tax rate from 46% to 34% . . . which equates to a 40% income tax rate for 1987 and a 34% tax rate prospectively to flow to the customers of Oklahoma." The case was then assigned to a hearing officer for the express purpose of "taking evidence, hearing and reporting thereon to the Commission." The hearing, which was initially set for December 7, 1987, came to be postponed many times because several parties had intervened, including one of the appellants, American Association of Retired Persons (AARP). It eventually took place on January 26, 27, 30 and February 3, 1989.

A myriad of witnesses testified on behalf of the State, SWB, AARP and Staff. In addition to live testimony, several parties offered "pre-filed testimony," which consists of written responses to questions prepared in advance of the hearing. These statements, most of which are accompanied by supporting documentation, were introduced and admitted as exhibits. After the hearing each party tendered its own proposed findings of fact and conclusions of law. The hearing officer adopted Staff's recommendations in toto.

Based upon its own assessment of the evidence adduced at the hearing, the Commission adopted nearly all of the findings made by the hearing officer. The Commission findings which are pertinent to the issues tendered in this appeal are: (1) the Tax Reform Act of 1986 reduced the federal corporate income tax rate from 46% to 34% and caused SWB to accumulate $30,677,167.00 in surplus cash between January 1, 1987 and September 30, 1989, (2) "Staff appropriately considered [other] known and measurable changes in SWB's business operations in Oklahoma, (3) a $180 million deficiency exists in SWB's depreciation reserve, (4) Staff was correct in applying October 1986 levels of "separations factors" when calculating SWB's rate base, (5) severance pay expenses were correctly excluded from the determination of SWB's revenue requirements, (6) in calculating SWB's "cash working capital," Staff correctly excluded "local service revenue billed in advance" and (7) interest should not be charged against SWB's excess deferred income taxes.

In accordance with these findings the Commission ordered that (a) interest is to be applied to SWB's surplus cash (or revenue excess) at the annual rate of 11.589%, (b) the revenues generated by tax law changes should not be refunded to ratepayers but instead are to be used for upgrading service, i.e., converting multi-party line areas to single-party service and modernizing central offices, (c) SWB must "file tariffs and rate schedules consistent with this order," implement the service improvement programs and rate reductions and submit a "private letter ruling request consistent with the purposes and findings set forth herein" and (d) "as a matter of policy" the depreciation reserve deficiency is to be amortized over a four-year period to provide "rate stability" for Oklahoma ratepayers.

The State, AARP and SWB each seek corrective relief from various portions of the Commission's order. For the reasons to be stated we affirm in part, reverse in part and remand this cause for further proceedings.

II.

AARP HAS STANDING TO APPEAL

SWB argues that AARP lacks standing to seek corrective relief because it is not aggrieved by the Commission's order. We are told that the State, through the Attorney General's office, and the Commission's Staff sufficiently protect "AARP's interest . . . to the same extent [that the interests of] . . . other ratepayers [are protected]." SWB claims "[t]here is no distinct injury to AARP" and the Commission's order now before us "cannot possibly cause injury, because it not only confers a benefit on Oklahoma ratepayers by ordering expenditures of funds for service improvements, it actually reduces mileage charges and rates for MTS and WATS services." We are not persuaded by these arguments. As SWB points out in its own brief, the terms of Art. 9 § 20, Okla. Const., explicitly provide that an appeal may be brought by "any party affected" or "any person deeming himself aggrieved" by any action of the Commission affecting the rates of a public service corporation. AARP qualifies as a "party affected" by the order now before us.

AARP's membership is composed of individuals who presumably are ratepayers. SWB does not contend to the contrary. Inasmuch as AARP, on behalf of its membership, had unsuccessfully pressed the Commission for a refund of SWB's surplus cash, AARP certainly may deem itself aggrieved or affected by the Commission's order, which impacts upon telephone service as well as rates. AARP doubtless has standing to press for relief in this appeal.

III.

A PUBLIC SERVICE CORPORATION'S SURPLUS CASH GENERATED SOLELY BY CHANGES IN INCOME TAX LAWS NEED NOT BE REFUNDED TO RATEPAYERS

SWB, a public service corporation, undisputedly accumulated over $30 million in surplus funds, which are attributed solely to the federal income tax law change. The State and AARP contend that this sum should be treated as an "overcharge" within the meaning of 17 O.S. 1981 § 121[ 17-121] and hence refunded to Oklahoma ratepayers. Although § 121 does expressly require that overcharges be refunded, the question is whether the surplus cash in controversy resulted from "charges. . . in excess of the lawful rate." We hold it did not.

The rates charged by SWB during the period in question (January 1, 1987 to September 30, 1989) clearly were authorized by the Commission. Inasmuch as SWB's accumulation of surplus funds is attributed solely to a decrease in the federal corporate income tax rate, this money was not obtained by overcharging ratepayers. Neither may the Commission's finding regarding the federal tax law's effect upon SWB be regarded as a declaration that a different rate should have been charged. In short, § 121 affords no authority for requiring the refund sought by AARP and the State.

IV.

SWB IS ACCOUNTABLE FOR INTEREST ON THE SURPLUS CASH IN SUIT

SWB urges that the Commission erred in charging or holding it accountable for interest on the surplus cash generated by the decrease in the federal corporate income tax rate. We reject this contention. The briefs contain no authority which militates in favor of reversing the Commission's decision. It is beyond question that SWB had the use of the funds whose availability stands attributed to a tax law change alone and which came to SWB without consideration of its rate base. "The presumption in the business world . . . and [in] equity, is that the use of money normally calls for the payment of interest." We find this rule to be applicable in this case.

V.

SWB'S SURPLUS CASH MAY BE USED TO ELIMINATE MULTI-PARTY LINES IN THIS STATE

Having held that SWB need not refund its surplus cash to ratepayers, the next question to be answered is whether the Commission erred by directing SWB's use of those funds to upgrade telephone service. We answer in the negative.

The record leaves no doubt that the Commission viewed SWB's accumulation of surplus revenue as presenting a "unique opportunity to accomplish service and central office upgrades in a manner which eliminates any adverse impact on basic exchange rates and any burden on ratepayers." The Commission's decision in this regard was hence one of "policy," and this court is not free to disturb that ruling if it is supported by "substantial evidence."

The Commission's efforts to replace multi-party lines with single-party connections are complemented by the "impossibility of identifying specific ratepayers" who would be entitled to a refund. Indeed, it would be inequitable to distribute funds based on some artificial formula that lacks a relation to the ratepayers' entitlement. In any event, there can be no doubt that the service improvements ordered by the Commission are inherently beneficial and supported by substantial evidence. On this record we are not free to block the Commission from pursuing its "goal of universal service by implementing [this improvement program]."

VI.

THE DECISION TO UPGRADE CERTAIN CENTRAL OFFICES IS NOT SUPPORTED BY SUBSTANTIAL EVIDENCE

The Commission approved the expenditure of a portion of SWB's surplus cash for upgrading fourteen "central offices" listed in one of Staff's exhibits. The State and AARP contend that the decision to improve these particular facilities lacks evidentiary support. We agree.

None of the parties challenges the Commission's finding that "[c]entral [o]ffice upgrades will improve the quality of service . . . and increase the level of technology available to customers. . . ." Nor is there any clear denial that modernization is generally needed. The fatal weakness in the Commission's choice of facilities lies in undisputed testimony that Staff's "Project Priority List" — upon which the Commission relied — was presented as "simply a listing of projects that one might consider." Staff's list of priority upgrade projects differed from that of SWB. Moreover, a witness for Staff even understood that a separate hearing would be held to evaluate the central offices slated for upgrading. In light of these admissions, it is no wonder that the Commission's order contains no reference to any specific information that could provide a basis for upgrading the listed central offices.

The Commission's order regarding the upgrade of Staff's proposed list of central offices hence cannot be sustained. A hearing should be conducted on remand to determine specifically which central offices are most in need of improvement or modernization.

VII.

THE COMMISSION'S ENLARGEMENT OF ITS INQUIRY TO INCLUDE CONSIDERATION OF CHANGES IN SWB'S BUSINESS OPERATIONS IN THIS PROCEEDING DOES NOT CONSTITUTE REVERSIBLE ERROR

AARP complains that since the proceeding before the Commission had been initiated solely to inquire into the impact of the Tax Reform Act of 1986 upon Oklahoma public utilities, the Commission impermissibly expanded the scope of its probe when it allowed SWB to present evidence of changes in its business operations. We disagree. The error urged here was not accompanied by a showing that AARP had been prejudiced by the inquiry's expansion. AARP makes no showing of this kind.

AARP urges that the Commission erred by failing to disregard "known and measurable changes in SWB's business operations." Had SWB's accumulation of surplus cash not been attributable to a tax law change but instead was caused by some other factor, i.e., a significant drop in the price of "energy", SWB would still benefit from the surplus. Under those circumstances as well as under the facts in this case it would be inherently unfair for the Commission to consider the surplus income in total separation or isolation of other utility needs.

In any event AARP has failed to show that the issues joined by SWB are not intertwined with the other elements which bear upon the rate structure. The scope of the Commission's inquiry was sound from legal, accounting and economic points of view. We hence conclude that the Commission did not abuse its discretion by considering evidence of changes in SWB's business operations in conjunction with its inquiry's original stated purpose.

VIII.

THE COMMISSION'S FINDING OF A DEPRECIATION RESERVE DEFICIENCY LACKS SUBSTANTIAL EVIDENTIARY SUPPORT

The State's prime contention in this appeal, which is also included in AARP's assignments of error, is that the Commission's finding of a deficiency in SWB's depreciation reserve not only lacks any foundation in substantial evidence but also is unsupported by any evidence at all. Among the "known and measurable changes" pertaining to business operations, the Commission considered SWB's depreciation reserve and found that a deficiency exists in the amount of $180 million. For the reasons to be stated, we reverse this portion of the Commission's order.

"Depreciation reserve" refers to

"an accounting technique whereby a fund is built up from annual contributions, as an item of expense of operation, over a period of time representing the service life of a public utility plant to offset and to equal in value the ultimate total loss through use of the utility property so that at the end of such service life the depreciation reserve fund will replace the property so worn out by the various factors of depreciation." (Citation omitted.)

A deficiency in the depreciation reserve is hence generated when the useful life of the property is unexpectedly reduced. In the words of the Commission a depreciation reserve deficiency "arises when depreciation rates previously established for plant and equipment become inadequate due to technological or physical obsolescence of the plant and equipment." In the scheme of rate structure, depreciation is an allowance given to SWB, i.e., an operating expense, by which the cost of an investment may be recovered through the rates charged.

If, for example, the useful life of an item of equipment had initially been predicted to be 10 years, but its life expectancy is later found to be no more than 6 years, then the rate base would be reduced faster than initially planned. The result is an increase in current operating expenses, which could warrant an increase in rates charged to customers. As an asset depreciates over time in accordance with a schedule ( or rate) that is based on an estimate of the asset's useful life, the rate base is reduced. A reduced rate base, in turn, means a decreased return on the investment. In short, when equipment becomes either less useful or obsolete, the schedule of depreciation becomes inadequate. This, in turn, gives rise to a deficiency in depreciation reserve. The question for us to resolve here is whether the Commission's finding that SWB has a depreciation reserve deficiency is supported by substantial evidence. This we answer in the negative.

The record reveals, and there is no question, that the Commission's reserve deficiency finding is based upon the testimony of a single Staff witness. According to the Commission's order, "[t]he amount of the depreciation reserve deficiency was established for SWB at the 1986 Three-Way Meeting wherein SWBT, Commission Staff and the FCC [Federal Communications Commission] staff met together for the purpose of discussing depreciation rates for SWBT." (Emphasis added.) Staff's witness "fully participated in this three-way process . . . [where] Staff reviewed the depreciation rates for each account and the reserve imbalance which the FCC had verified to exist through the use of computer programs which analyzed data supplied by SWB. Staff's calculations took into account a total state reserve deficiency of $180 million." (Emphasis added.) So noted the Commission.

As a result of this meeting the FCC determined that a depreciation reserve deficiency exists. Although the Commission may have, in SWB's words, "made its own independent decision" regarding reserve deficiency, we conclude that the ruling at hand consists of no more than a decision to agree with the FCC's determination that a deficiency exists.

SWB admits in its brief that "Staff [had] recommended to the Commission the depreciation reserve deficiency amount only after being furnished with and reviewing capacious records related to SWBT in Oklahoma. This process included presentation of voluminous depreciation data (binders) to the FCC. . . ." (Emphasis added.) Although SWB relies heavily upon the notion that this evidence is undisputed, none of this data was tendered for the Commission's own consideration. It is also undisputed that the Commission relied upon its Staff's participation in the Three-Way Meeting and that its Staff had there relied upon the FCC's assessment of data and records which SWB had voluntarily supplied. In addition, the Commission approvingly observed in its order that " [S]taff had the opportunity to review the audit conducted by the FCC and was satisfied concerning its accuracy." (Emphasis added.)

Staff urges that its witness provided expert testimony upon which the Commission could rely in making its factual determination that a depreciation reserve deficiency exists. We reject the notion that there is an "evidentiary" foundation upon which the Commission's finding is grounded, since it considered no specific data or documentation that could satisfy the "substantial evidence" standard.

The evidence upon which the Commission based its finding consists of expert testimony about data dehors the record. This is no substitute for the Commission's own assessment of evidence before it. Indeed, not even the common law's "summary rule," now codified in 12 O.S. 1981 § 3006[ 12-3006], can cure this defect in proof. The summary rule permits an expert to summarize a mass of data and reports before the court. Testimony of this nature is admissible even though its basis, i.e., the mass of materials, has not been admitted into evidence.

Had the summary rule been invoked by the evidence's proponents, the raw data's availability would have been required for inspection by the opposing party. Without question the raw data upon which the witness relied here was neither tendered nor made available otherwise in the course of proceedings before the Commission. In short, because the summary rule's sound application presupposes the existence of an underlying pool of raw data before the tribunal and hence available for examination during the proceeding — and nothing less will suffice — it affords no safety net for the Commission's finding.

Nor do the terms of 75 O.S. 1981 § 310[ 75-310](4) dispense with the need for either the Commission's own assessment of data or the latter's availability at the proceeding. Section 310(4) provides that "notice may be taken of generally recognized technical or scientific facts within the agency's specialized knowledge." Although the terms of this section of the Administrative Procedures Act, 75 O.S.Supp. 1990 § 250[ 75-250] et seq., do not apply to the Commission, the minimum standards of due process embodied in § 310(4) nonetheless do. In any event, the record does not indicate the Commission, however informally, took "notice" of the FCC's findings and invited the opposing parties to meet the extra-record material by rebuttal. Rather, the Commission simply adopted a mass of proof without the benefit of a proper predicate. Moreover, the extra-record "facts" described by the testimony as supportive of the depreciation reserve deficiency finding would not necessarily qualify as "generally recognized technical facts."

IX.

THE COMMISSION SHOULD HAVE CONSIDERED SWB'S SEVERANCE PAY EXPENSES

Among the factors SWB pressed for consideration by the Commission was severance pay expense. Although Staff had earlier agreed with SWB that $4.7 million is an appropriate sum to be included in calculating the utility's revenue requirement, Staff nevertheless excluded it from their proposal. The Commission also chose to disallow SWB's severance pay expense. On this point we reverse.

According to the Commission's order, Staff's treatment of severance pay was correct " because Staff was unable to verify annualized wage levels due to the methodology that SWBT uses to track its Oklahoma labor costs." (Emphasis added.) This explanation, though uncontroverted, leaves us with no doubt that the Commission's decision in this regard is arbitrary and hence cannot be sustained.

The basis for disallowing severance pay expense is not questioned by any party. The State, AARP and Staff all agree that SWB either failed or was unable to supply Staff with a "labor-tracking mechanism," that is, audit information for allocating wage and non-wage data between state and interstate jurisdictions. While SWB does not deny this, none of its opponents has offered any explanation as to how an allowance for severance pay expenses is dependent upon a labor-tracking audit. Neither have we been offered a reason to doubt SWB's assertion that

"the breakdown of each regulatory account between wage and non-wage data does not mean that SWBT has non-verifiable or non-auditable severance pay expenses. Total payroll expenses for Oklahoma employees, and total severance pay expenses are easily audited, and were verified by Staff. The verifiability of these expenses does not depend on being able to go to the regulatory account for maintenance expenses, for example, and break down that account between wage and non-wage data. That type of breakdown has nothing to do with the keeping of records relating to severance pay, which Staff in fact audited."

SWB's failure or refusal to produce records, data or other information can make it subject to the Commission's contempt or other coercive powers. These circumstances are no substitute for having substantial evidence in support of an order. The Commission's disallowance of SWB's severance pay expense must hence be disapproved on the present record, subject to further consideration on remand.

X.

THE COMMISSION MISCALCULATED SWB'S CASH WORKING CAPITAL

The next issue involves the Commission's computation of SWB's "cash working capital" requirement. Cash working capital refers to the amount of cash needed from the utility's own funds to meet current obligations as they arise during the lag period between services rendered and collection of revenues. Using a "modified formula approach" the Commission found that SWB had a negative cash working capital requirement, which indicates the utility needs no cash to sustain its operations between the time services are rendered and the time payment for them is received. SWB contends the opposite — that it has a positive cash working capital requirement and hence needs cash during this period. SWB claims that its conclusion is based upon the Commission's own "modified formula approach," while the State, AARP and Staff maintain that it is based upon a flawed "lead-lag" study. SWB submits that the only reason it offered the lead-lag study was because its availability was mandated by the Commission during SWB's previous rate-making case.

According to SWB, the Commission's calculations are flawed because Staff based its approach on an "instant receipt" assumption — namely that revenues are received on the date of billing. We are told this assumption is erroneous because (1) bills are not mailed to customers until one week after the billing date and (2) there is a lag period of approximately twenty-five days from the date of the bill to the date of payment.

While it is not clear from the record whether Staff actually used the "instant receipt" assumption in its calculations, Staff's approach at least appears to lack a basis in substantial evidence. SWB does not challenge Staff's use of the "modified formula approach," though the Commission did fail to specify the particular factors it employed. Neither did it state whether it considered the lag in rendition of services and receipt of payment therefor. Moreover, the briefs direct us to no evidence of how Staff's calculations resulted in a negative cash working requirement, while SWB, using the same "modified formula approach," arrived at a positive cash working capital requirement. Because the Commission's determination of SWB's cash working capital requirement is not sustained by substantial evidence, we disapprove of this part of its order and remand the issue to the Commission for further consideration.

XI.

THE COMMISSION'S DECISION NOT TO CONSIDER INTEREST ON SWB'S EXCESS DEFERRED INCOME TAXES WILL NOT BE DISTURBED

The State and AARP contend that the Commission erred by failing to charge interest on SWB's excess deferred income taxes. Because the briefs fail to provide us with a record-supported argument that would defeat the basis for the Commission's decision, we must affirm.

The hearing officer's report explains that "[e]xcess deferred income taxes are the result of timing differences between tax expenses charged for books and taxes payable to the state and federal governments. Any differences are accumulated in the deferred income tax balance and are deducted from rate base until the taxes are actually paid to the government."

Here, SWB collected from the ratepayers sums sufficient to satisfy the 46% income tax rate but paid taxes for 1987 at 40% and at 34% in 1988 and the prospective year. An excess is thus created because the deferred tax balance became greater than the amount of taxes actually paid. The Commission decided that it would not be appropriate in this case to charge interest for SWB's excess deferred income taxes. This is so because ratepayers are benefited when the excess is deducted from the rate base, thus lowering the revenue requirement. According to the hearing officer's report, charging this interest would result in a substantial revenue deficiency, and consequently, the need to raise telephone rates.

The AARP contends that this decision is erroneous because, in their view, "[i]f the Commission would immediately refund to the ratepayers who paid the deferred income taxes, these ratepayers could immediately deposit the money in the bank and draw interest on it." The AARP also contends that there is no substantial evidence the ratepayers will receive any benefit by SWB's retention of the excess deferred income taxes, as the Commission has permitted.

Every finding of the Corporation Commission made in matters it frequently adjudicates and in which it possesses expertise is accompanied by a presumption of correctness if supported by substantial evidence. Although AARP did present expert testimony favoring its position, the Commission's decision is nonetheless supported by substantial evidence. Moreover, upon this point AARP's brief offers no clear basis for overcoming the law's presumption of correctness that attaches to the Commission's decision. We hence leave that part of the order undisturbed.

AFFIRMED IN PART AND REVERSED IN PART; CAUSE REMANDED FOR FURTHER PROCEEDINGS NOT INCONSISTENT WITH THIS PRONOUNCEMENT.

LAVENDER, SIMMS, DOOLIN, HARGRAVE and ALMA WILSON, JJ., concur.

KAUGER and SUMMERS, JJ., concur in result.

HODGES, V.C.J., disqualified.