40 ILCS 5/13-504
All reserves and liabilities for annuities under this Article shall be computed according to actuarial tables adopted by the Board.
At least once every 5 years a valuation shall be made by the actuary of the liabilities and reserves of the Fund, including a general investigation of the mortality, retirement, employment turnover, interest, and earnable compensation experience of the Fund, and a report thereon shall be made to the Board. The actuary shall recommend to the Board such actuarial tables and rates of interest as are required in the operation of the Fund.
For computing retirement and surviving spouse's annuities, all employee contributions in the form of salary deductions or otherwise, and all concurrent contributions by the District shall be improved in an amount equal to 3% per annum from the date contributions become due or have accrued to the date an annuity is applied for and becomes payable.
40 ILCS 5/13-504