Unlike simple random sampling, quota sampling selects subjects one at a time until desired percentages are reached.
Polls of the 1948 U.S. presidential election illustrate an example of quota sampling. Respondents were chosen
according to gender, age, income, education, and factors related to political views. However, the polls
underestimated the popularity of Harry Truman and overestimated the popularity of Thomas E. Dewey because
Republicans were over represented in the poll. It is impossible to control for all variables in quota sampling.