Thus, productivity tends to be higher in urban areas, and agglomeration reinforces the comparative advantage of cities. In addition to such advantages, urban residents have, on average, better access to education and health care - as well as to other basic services such as safe drinking water, basic sanitation, transportation and communication -than rural populations. For instance, in developing countries, 73 per cent of urban dwellers, and only 33 per cent of the rural population, had access to basic sanitation facilities in 2004 (WHO and UNICEF, 2006). Salaries and returns to assets are often higher in urban areas because of higher productivity, as well as better infrastructure and services. According to the World Bank, 75 per cent of those living in extreme income poverty resided in rural areas in 2002, despite the fact that only about 52 per cent of the world population were living in such areas (Ravallion, Chen and Sangraula, 2007).