Question: In each of the following situations, specify the population. Also, state the two categorical variables that would be measured for each unit in the sample and the two categories for each variable.
a. Researchers want to know if there is a relationship between having graduated from college or not and voting in the last presidential election, for all registered voters over age 25.
b. Researchers want to know if there is a relationship between smoking and divorce for people who were married during the 1970s and 1980s.
c. Researchers classify midsize cities in the United States according to whether the city's median family income is higher or lower than the median family income for the state in which the city is located. They want to know if there is a relationship between that classification and proximity of the city to a major airport, defined as whether or not one of the 30 busiest airports in the country is within 50 miles of the city.