This map shows how commercial activity is distributed within urban areas and the impact of commercial services on the urban landscape, by mapping what proportion of stores (hence jobs) in an urban area that are found in shopping centres. Shopping centres are designed, built and managed as a single unit, primarily for retail purposes; they are therefore easy to identify. As a relatively modern innovation, introduced to most cities in the 1960s or later, they are usually located at the edge of the city closer to the suburbs. Most cities have about 10% of their stores in shopping centres; this value is slightly higher in larger cities and in cities with a high growth rate.