California Code, Government Code - GOV § 20042

On the election of a contracting agency, other than a county superintendent of schools with respect to a contract under Chapter 6 (commencing with Section 20610), “final compensation” for a local member employed by that agency whose retirement is effective or whose death occurs after the date of the election and with respect to benefits based solely on service credit accrued with the contracting agency making the election under this section shall be computed under Section 20037 but with the substitution of the period of one year for three consecutive years.  An election under this section shall be made by amendment to the contracting agency's contract made in the manner prescribed for approval of contracts, except that an election among the employees is not required, or by express provision of the contract if exercised at the time of approval of a contract.