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 (either partner smokes) and divorce for people who were married between 1990 and 2000.
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 airport availability, defined as whether or not one of the 30 busiest airports in the country is within 50 miles of the city.