Average hourly earnings reflect not only changes in basic hourly and incentive wage rates but also such variable factors as premium pay for overtime and late- shift work and changes in input of workers paid on incentive plan. They also reflect shifts in the number of employees between relatively high-paid and low- paid work and changes in workers_ earnings in individual establishments. Averages of hourly earnings differ from wage rates. Earnings are the actual return to the worker for a stated period of time; rates are the amount stipulated for a given unit of work time. The earnings series do not measure the level of total labor costs on the part of the employer since the following are excluded: Irregular bonuses, retroactive items, payments of various welfare benefits, payroll taxes paid by employers, and earnings for those employees not covered under production worker, construction worker, or nonsupervisory employee definitions.