In his 1978 textbook for undergraduates, Urban Economics, Harry W. Richardson, an economics professor at USC Los Angeles, called for a new model through which to consider the phenomenon of subcentering. Subcentering is Richardson's term for the development of areas of the city away from the CBD (central business district) that have high concentrations of "non-residential establishments." Distance from the downtown core solves potential problems of competition and crowds. Their existence raises questions such as what the ideal distance from the CBD would be, why they arise, and how they affect overall urban spatial organization. They often arise where old towns and villages once stood, before they became part of the city. Alternatively, a subcenter could develop close to a new "supra-threshold foci of activity" such as a hospital, school, or transportation hub. The effect of a subcenter on housing rates depends on the level of wealth that prevails in the city. Poorer populations might tend to live near the subcenter that serves their needs and provides employment. In wealthier cities, people can afford to be more flexible in terms of where they live. The population density gradient is used as an indicator of urban population patterns.