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 “advance 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 of these 286 responded to the telephone survey. Th e other 367 households were assigned to receive only the advance letter, and of these 245 responded to the telephone survey.
a. Is this an experiment or an observational study? Explain how you are deciding.
b. Organize the counts in a well-labeled 2 × 2 table.
c. Find the observed difference in the conditional proportion of households that responded to the telephone survey between those who received an incentive and those who didn’t receive an incentive.
d. Give two possible explanations for the observed difference in conditional proportions as reported in part (c).