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Question: a study is conducted to investigate the association between chewing...

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A study is conducted to investigate the association between chewing tobacco and oral leukoplakia (a precancerous lesion) among currently active professional baseball players in the southeastern United States. A roster of all active players is obtained (n=500). All potential study subjects agree to participate. Each subject has an interview regarding current use of chewing tobacco and has his mouth examined by a dentist. Of the 500 subjects, 125 subjects chew tobacco and 375 do not chew tobacco. Of the chewers, 25 have evidence of oral leukoplakia. Of the non-chewers, 15 have evidence of oral leukoplakia. All 500 players were followed for a period of 5 years. Of those who had evidence of oral leukoplakia, 18 died of some type of oral cancer.

  1. Draw a 2 x 2 table demonstrating the relationship between chewing tobacco and oral leukoplakia. In drawing this table, put the exposure variable on the columns and the health outcome variable on the rows.
  2. Draw a second 2 x 2 table demonstrating the relationship between chewing tobacco and oral leukoplakia, but this time, put the exposure variable on the rows and the health outcome variable on the columns.
  3. Using the table drawn in part a, compute the prevalence ratio and the prevalence odds ratio of oral leukoplakia for chewers compared to non-chewers. Are these two estimates close to one-another? Why are these prevalence measures and not incidence measures?
  4. Using the table drawn in part b, compute the prevalence ratio and the prevalence odds ratio of oral leukoplakia for chewers compared to non-chewers. Are these estimates equal to their corresponding estimates computed using the table drawn in part a? Explain.
  5. Ignoring the issue of statistical inference and the control of other variables, what do these results say about the relationship between chewing tobacco and the presence of oral leukoplakia?
  6. Calculate the case-fatality rate (actually, a risk) in this study. Why is this a measure of risk?
  7. Based on the information provided, why can’t you evaluate whether tobacco chewers have a higher case-fatality risk than non-chewers?
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