For the purposes of Section 23817.5, beginning with the year 2000, population shall be determined by the most recent United States decennial census or a single subsequent census between United States decennial censuses validated by the Population Research Unit of the Department of Finance five years after a United States decennial census.
Source:
Section 23817.9, https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=BPC§ionNum=23817.9..