18 Active vs passive
Case based surveiallance systems can be divided whether they are active or passive. Many surveillancesystems have elements of both.
18.1 Active surveillance system
An active surveillance system involves a group of individuals who actively collect information for the surveillance system.
An example of an active surveillance system would if the staff of an agency goes door-to-door to gather information.
18.2 Passive surveillance system
A passive surveillance system uses data collected for other purpose. Passive surveillance can be seen as secondary data analysis.
An example for a passive surveillance system could be a system that extracts data from a hospital database.