The selection of a DSS site depends on several considerations. Some sites emerge as a result of specific intervention trials resulting in the creation of resources that make it easier to develop into a site. However, there are other considerations, such as whether there is interest in a rural or urban setting. Whatever the genesis is, the site should have a geographically defined area or areas that will allow for the population of that area to be monitored. Some sites may have the advantage of being located within clearly defined areas, in which case they may have pre-existing maps and populations that can be easily sourced from the national population offices. There is no ideal population size that should constitute a site, but most DSS sites cover an average population of about 70,000 which is ample to track trends in most common causes of mortality. The area to be covered by a DSS site may be based on a cluster of sub-districts or localities or they may be based on a larger single geographical area such as a clearly defined district.  Sites should be selected so that households can be reached in all reasonable weather in all seasons.