Methods According to the program for controlling birth defects in Shanxi province, our county has carried out the continuous surveillance of birth defects since 1997's retrospective investigation.

Rates and causes of child mortality were obtained from a continuous-registration demographic surveillance system (DSS) in Malawi for a population of 32 000.

The challenge is multiplied when the surveillance needs to be continuous, which 'makes non-stealthy slow-speed aircraft easier to detect'.