One factor contributing to neighbourhood distinctiveness and social cohesion in past cities was the role of rural to urban migration. This was a continual process in preindustrial cities, and migrants tended to move in with relatives and acquaintances from their rural past. Neighbourhood sociology is a subfield of urban sociology which studies local communities Neighbourhoods are also used in research studies from postal codes and health disparities, to correlations with school drop out rates or use of drugs. Neighbourhoods have been the site of service delivery or "service interventions" in part as efforts to provide local, quality services, and to increase the degree of local control and ownership. Alfred Kahn, as early as the mid-1970s, described the "experience, theory and fads" of neighbourhood service delivery over the prior decade, including discussion of income transfers and poverty. Neighbourhoods, as a core aspect of community, also are the site of services for youth, including children with disabilities and coordinated approaches to low-income populations. While the term neighbourhood organisation is not as common in 2015, these organisations often are non-profit, sometimes grassroots or even core funded community development centres or branches. Community and economic development activists have pressured for reinvestment in local communities and neighbourhoods. In the early 2000s, Community Development Corporations, Rehabilitation Networks, Neighbourhood Development Corporations, and Economic Development organisations would work together to address the housing stock and the infrastructures of communities and neighbourhoods (e.g., community centres). Community and Economic Development may be understood in different ways, and may involve "faith-based" groups and congregations in cities. In the 1900s, Clarence Perry described the idea of a neighbourhood unit as a self-contained residential area within a city. The concept is still influential in New Urbanism. Practitioners seek to revive traditional sociability in planned suburban housing based on a set of principles. At the same time, the neighbourhood is a site of interventions to create Age-Friendly Cities and Communities (AFCC) as many older adults tend to have narrower life space. Urban design studies thus use neighbourhood as a unit of analysis. In mainland China, the term is generally used for the urban administrative division found immediately below the district level, although an intermediate, subdistrict level exists in some cities. They are also called streets (administrative terminology may vary from city to city). Neighbourhoods encompass 2,000 to 10,000 families. Within neighbourhoods, families are grouped into smaller residential units or quarters of 100 to 600 families and supervised by a residents' committee; these are subdivided into residents' small groups of fifteen to forty families.