40 ILCS 5/12-106
"Actuarial tables": The American Experience Table of Mortality and 4% interest for any present employee or future entrant who was a participant or contributor to the fund on June 30, 1959, and for their widows or other beneficiaries, except as to reserves on annuities for the computation of which the Combined Annuity Mortality Table and 4% interest shall be used, and the Combined Annuity Mortality Table and 4% interest for any future entrant whose first participation in the fund began on or after July 1, 1959, and for his widow or other beneficiaries.
All annuities and reserves on annuities, present or prospective, except as may otherwise be provided, shall be computed according to such actuarial tables and regular interest, as herein defined: provided, however, that effective as of July 1, 1979 the actuarial table and rate of interest to be used shall be that adopted by the retirement board upon recommendation of the actuary.
40 ILCS 5/12-106