Urban DSS sites are usually less costly since access to large populations in relatively small areas is much easier than in rural DSS sites.  However urban DSS sites have additional challenges, the most important being the high degree of mobility of the population.  This necessitates more frequent update rounds in some cases.  Other problems, especially in the poorest parts of urban settings is that large numbers of the population may live in unofficial settlements.  With the rapid influx of rural populations to urban areas, such new urban residents maintain many close links to rural communities and may be regularly exposed to health and disease risks in both settings.  On the peri-urban edge of cities, the same environment may exemplify both urban and rural realities simultaneously.  Usually urban DSS requires more sophisticated database systems to accommodate the dense and transient nature of the population.