Dr. Simpson wants to investigate opinions favoring/opposing federalized healthcare among adults living in cities ranging from 100,000 and 250,000 people. First, Dr. Simpson randomly selects 10 cities in the U. S. that have a population between 100,000 and 250,000. Second, he randomly selects 10 census tracts from each of the 10 cities (a total of 100 census tracts). Third, Dr. Simpson selects two blocks from each census tract (a total of 200 blocks). Fourth, he randomly selects 5 houses from each block (a total of 1000 households). What kind of sample is Dr. Simpson using