As of June 30, 1991, and thereafter at the end of periods not to exceed four years, the actuary shall make an actuarial investigation into the mortality, service, and compensation experience of members and persons receiving benefits and an actuarial valuation of the assets and liabilities of this system. From time to time, the actuary shall determine the rate of interest being earned on the retirement fund after deducting from earnings amounts applied to costs of administration of this system.