(a) As used in this chapter, the term "annual payroll" means the total amount of wages for employment paid by an employer during the 12 month period immediately preceding and ending on the computation date.

(b) As used in this chapter, the term "average annual payroll" means the average of the annual payrolls of an employer for the last three 12 month periods immediately preceding the computation date, except that for an employer whose account could have been charged with benefit payments throughout at least 12 but less than 36 consecutive calendar months immediately preceding and ending on the computation date, the term "average annual payroll" means the total amount of wages for employment paid by such employer during the 12 month period immediately preceding and ending on the computation date.