Between August 1973 and September 1974, 957 randomly selected adults in Washington County, Maryland, were interviewed about whether they wore seat belts and whether they had undergone a dental checkup in the past two years. Their responses are summarized in Table 23. During that period, no state required car occupants to wear seat belts.

Suppose one individual from the study is randomly selected.
a. Find P (wear seat belt | dental checkup within two years).
b. Find P (wear seat belt | dental checkup more than two years ago).
c. Compare the results you found in parts (a) and (b). What does your comparison mean in this situation?
d. On the basis of the results you found in parts (a) and (b), a student concludes that undergoing frequent dental check- ups causes a person to wear a seat belt. What would you tell the student? Include a possible lurking variable in your response.
e. The sampling was done by randomly selecting a house- hold in a frame of all the households in the county and then selecting every kth household. An individual to be interviewed was randomly selected from each household. Identify the sampling method used to select the households.
f. Some individuals were not included in the study because they did not own cars. Other individuals refused to participate in the study. Individuals who participated in the study self-reported whether they wore seat belts and when they last had a dental checkup. Identify likely type(s) of bias.

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Table 23 Wore a Seat Belt versus Under- went a Dental Checkup Wear Seat Belt Dental Checkup Within past two years More than two years ago90 107 197 Yes No Total 428 332 760 Total 18 3 957