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Confounding


Problems

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1. Olsen (1991) reported data concerning the relationship between smoking and ability to get pregnant. Coffee and tea consumption is a potential confounder. Some of the data is shown below.
              0-3 Cups of Coffee        > 7 Cups of Coffee
                or Tea per Day           or Tea per Day

   Cigarettes  Pregnant in Six Months    Pregnant in Six Months
   Per Day         Yes      No               Yes      No
      0           1854     407               381      86
   GT 19            55      20               190      70

a. Compute the crude odds ratio and the stratum specific odds ratios. Does coffee or tea consumption seem to be a confounder?
b. Use the chi square test of independence to determine if coffee or tea consumption is related to smoking level and to ability to get pregnant? Does coffee or tea consumption meet the definition of a confounder?
Olsen, J. (1991) Cigarette smoking, tea and coffee drinking, and subfecundity. American Journal of Epidemiology. 133(7), 734- 739.

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2. A resident (Haynes, et al, 1991) hypothesized that surgical residents who were sleep deprived as a result of being on call would have a higher rate of complications on surgeries that they performed than would those who were not on call. He collected call status concerning being on call the night before a surgery and presence or absence of a complication.
                    Complication
      On Call     No    Yes  % Complication
        Yes      1718   122     .066
        No       4435   237     .051
        Total    6153   359     .055

a. Conduct a test to determine if there is a relationship between call status and complication rate. What do you conclude?
Whether or not the surgery was an emergency is a potential confounder. Stratum specific data is shown below.
            Emergency              Not Emergency
           Complication             Complication
 On Call    No    Yes        On Call    No    Yes 
    Yes     624    79           No     1094    43 
    No     1049   121           Yes    3386   116
  Total    1673   200         Total    4480   159

b. Test the hypothesis that there is a relationship between call status and complication rate for each stratum. What do you conclude now.
c. Does emergency status meet the definition of a confounder?
Haynes, D., Schwedler, M., Dyslin, D., Rice, J. and Kerstein, M. (1995) Are postoperative complications related to resident sleep deprivation? Southern Medical Journal. 88(3), 283-289.

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