The beginning-of-the-year net assets amount was used in the ROI measurement because, in management's view, investment added during a given year resulted in little, if any, incremental profit in that year.
The investment would likely increase future profits.
Top management felt that such investments might not be proposed if were penalized (in the form of higher net assets and lower ROI) in the first year investment.
Because the investment base for the year was "frozen" at the beginning- of-the-year level, maximizing profit during the year was equivalent to maximizing ROI.
For beginning-of-the-year net assets, cash and receivables were allocated to plants on the basis of sales revenue, while inventories, property, plant, equipment, and current liabilities were traced specifically to each plant.
Historical cost less accumulated depreciation (book value) was used to value property, plant, and equipment.
The AM division's ROI was measured in the same manner as the plants' ROI.