California Code, Health and Safety Code - HSC § 44122

(a) Vehicle emissions shall be based on either direct testing, statistical sampling, or emission modeling methods. Sampling of a statistically significant portion of the vehicles may be used to estimate emission benefits or to develop and validate correlations for use in estimating emission benefits.

(b) A reasonably reliable mechanism shall be applied to estimate vehicle miles traveled and the remaining useful life of each purchased vehicle. The odometer reading shall be matched on each purchased vehicle with the records of the Department of Motor Vehicles and smog check records to verify driving history, or statistical data shall be used to estimate vehicle use.

(c) An annual survey shall be performed of a statistically meaningful number of participants to determine replacement vehicle and post-participation behavior and also to determine the extent, if any, of in-migration of low-cost vehicles due to price increases in the scrapping market area resulting from the scrap program.