Demographic surveillance systems
DSS are useful in countries where vital registration does not occur. They enable estimation of key demographic characteristics of the study population, such as fertility and mortality rates. Most DSS combine this with a research function which enables them to look at wider aspects of demography and health such as infectious diseases (HIV, malaria, TB), vaccination, nutrition (vitamin supplementation) and non-communicable diseases. These DSS are then referred to as Health and Demographic Surveillance Systems (HDSS). The INDEPTH network is an organisation that brings together community-based longitudinal demographic surveillance activities from different parts of the world. Their website contains profiles of the different HDSS that make up the network: http://www.indepth-network.org/index.php?option=com_content&task=view&id=1306&Itemid=1070 
Demographic surveillance systems (DSS) involve a large number of people followed over many years. Eligibility for the study is typically defined by residence and is thus geographical. Data are collected at regular intervals (usually every few months). The information requested at each visit may be quite minimal to facilitate frequent rounds of data collection and enable reporting by proxy respondents.