The history of urbanization focuses on the processes of by which existing populations concentrate themselves in urban localities over time, and on the social, political, cultural and economic contexts of cities. Most urban scholars focus on the "metropolis," a large or especially important city. There is much less attention to small cities, towns or (until recently) to suburbs. However social historians find small cities much easier to handle because they can use census data to cover or sample the entire population. In the United States from the 1920s to the 1990s many of the most influential monographs began as one of the 140 PhD dissertations at Harvard University directed by Arthur Schlesinger, Sr. (1888-1965) or Oscar Handlin (1915-2011). The field grew rapidly after 1970, leading one prominent scholar, Stephan Thernstrom, to note that urban history apparently deals with cities, or with city-dwellers, or with events that transpired in cities, with attitudes toward cities – which makes one wonder what is "not" urban history.