Ill. Admin. Code tit. 83 § 287.40
A utility may propose pro forma adjustments (estimated or calculated adjustments made in the same context and format in which the affected information was provided) to the selected historical test year for all known and measurable changes in the operating results of the test year. These adjustments shall reflect changes affecting the ratepayers in plant investment, operating revenues, expenses, and cost of capital where such changes occurred during the selected historical test year or are reasonably certain to occur subsequent to the historical test year within 12 months after the filing date of the tariffs and where the amounts of the changes are determinable. Attrition or inflation factors shall not be substituted for a specific study of individual capital, revenue, and expense components. Any proposed known and measurable adjustment to the test year shall be individually identified and supported in the direct testimony of the utility. Each adjustment shall be submitted according to the standard information requirement schedules prescribed in 83 Ill. Adm. Code 285.
Ill. Admin. Code tit. 83, § 287.40