See: Calculation of gross earnings
Earnings were measured according to different time periods: hourly, weekly/monthly, and annual. Calculation of these measures posed a number of problems because the longer the reference period, generally, the greater the number of employees whose pay is affected by absence or mobility. In order to calculate these different measures a number of different sub-populations or 'bases' were used. Their purpose was to keep bias and response burden to a minimum while maintaining the maximum possible level of transparency in the data for users.To permit the necessary calculations to be made, four bases were defined.
Eurostat, "Statistics on the structure and distribution of earnings: Methods and definitions", Data 1995, 1998 Edition, Office for Official Publications of the European Communities, Luxembourg, 1999, p.12