40 ILCS 5/11-123

Effective rate of interest, interest at the effective rate or interest.

"Effective rate of interest", "interest at the effective rate" or "interest": Interest at 4% per annum for an employee who was a contributor on January 1, 1952; and at 3% per annum for an employee who becomes a contributor after January 1, 1952. In all cases involving reserves, credits, transfers, and charges, "effective rate of interest", "interest at the effective rate" or "interest" shall be applied at these rates.