Recent legal action and research raise doubts about the advisability of measuring fiscal effort in the Illinois general grant-in-aid program by using the simple school district tax rate for operational purposes. Various logical, and perhaps legal, considerations might be sufficient to compel adjustment of the measurement of effort. Beyond these reasons, indications are that in the long run, greater state aid will go to districts that pass tax referenda and retain higher tax rates. The weight of research evidence suggests that low-income school districts will not be able to pass school tax referenda as often and therefore will not be able to take advantage of Illinois'"reward for effort" grant-in-aid system. An income factor is needed in the Illinois formula, more as a correction factor in the measurement of fiscal effort, then as an adjustment to "ability to pay." Introducing a new effort specification in the formula will require change in other parameters in that formula, including the guaranteed valuation level. If Illinois wishes to retain its present concept of "equal expenditure for equal effort," there must be a reevaluation of how effort will be measured. (Author/JG)