Also, your claim about what the study covers is wrong. From the paper:

"Our sample for this paper covers Baltimore, Boston, Brooklyn, Chicago, Cleveland,

Detroit, Manhattan, Philadelphia, Pittsburgh, and St. Louis. These were the largest nine northern cities in

1930, covering nearly 18 million people (about half of the total in the largest 100 cities) and about 38

percent of the urban black population living outside of the states of the former Confederacy."