A team of researchers (Singer et al., 2000) used the Survey of Consumer Attitudes to investigate whether incentives would improve the response rates on telephone surveys. A national sample of 735 households was randomly selected, and all 735 of the households were sent an €œadvance letter€ explaining that the household would be contacted shortly for a telephone survey. However, 368 households were randomly assigned to receive a monetary incentive along with the advance letter, and the other 367 households were assigned to receive only the advance letter. Here are the data on how many households responded to the telephone survey.

a. Was this an observational study or an experiment? How are you deciding?
b. What are the observational units?
c. What are the variables recorded? For each variable, identify the type of the variable (categorical or quantitative) and the role of the variable (explanatory or response).
d. Did the study involve random sampling? If yes, what is the advantage of a randomly selected sample? If no, what is the disadvantage?
e. Did the study involve random assignment to either receive an incentive or not? If yes, what is the advantage? If no, what is the disadvantage?
f. An appropriate analysis of the data shows there is evidence that people receiving an incentive are more likely to respond to the telephone survey compared to those receiving no incentive. Is it appropriate to conclude that incentives improve response rates? Why or why not? Explain your reasoning.

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Received an incentive? Yes Total No Responded to Yes 245 531 286 the telephone 82 122 204 No No survey? Total 368 367 735