# 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.

- 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.
- 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.
- 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?
- 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.
- 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?
- Calculate the case-fatality rate (actually, a risk) in this study. Why is this a measure of risk?
- Based on the information provided, why can’t you evaluate whether tobacco chewers have a higher case-fatality risk than non-chewers?