Two censuses were held in 1994 and 1998, in order to verify and supplement the data from the previous censuses. Censuses are planned every two years in order to have a clear picture of the study population at certain fixed points in time.
In the meantime, the population is monitored by a system for vital events registration. Previously conducted as a monthly activity, the vital event registration designed as a three-month cycle data collection system since January 2000.
All the households of the surveillance area are covered.