A prospective cohort study revealed a strong positive association between smoking and liver cirrhosis (relative risk = 2.8). The researchers then divided the cohort into two groups: alcohol consumers and non-consumers. Subsequent statistical analysis did not reveal any association between smoking and liver cirrhosis with either group. The scenario described above is an example of which of the following?

Question

A prospective cohort study revealed a strong positive association between smoking and liver cirrhosis (relative risk  = 2.8).  The researchers then divided the cohort into two groups: alcohol consumers and non-consumers.  Subsequent statistical analysis did not reveal any association between smoking and liver cirrhosis with either group.  The scenario described above is an example of which of the following?

Quizplus · Accepted Answer

Confounding refers to the bias that results when the exposure-disease relationship is mixed with the effect of extraneous factors (i.e., confounders). Confounders influence both the exposure and outcome. In the given study, crude analysis of the data initially revealed an association between smoking and liver cirrhosis. The most likely potential confounder is concomitant alcohol consumption in people who smoke. It is a well-known fact that alcohol consumption is strongly associated with liver cirrhosis. Furthermore, alcohol consumption can explain at least part of the association observed between smoking and liver cirrhosis. Methods to deal with confounding include matching of cases and controls based on the confounding factor, or stratification of the study population based on the confounding factor. In this case, running separate analyses for alcohol consumers and non-consumers (this technique is called stratified analysis) can unmask confounding and disclose the true unconfounded value of the RR. (Choice A) Selection bias results from the manner in which people are selected for the study, or from the selective losses from follow-up. The scenario does not mention any of these problems. (Choices B and C) Observer's bias and measurement bias distort the measure of association by misclassifying exposed/unexposed and/or diseased/non-diseased subjects. The scenario does not describe this classification process. (Choice D) Recall bias results from the inaccurate recall of past exposure by people in the study. It applies mostly to case-control studies, not cohort studies. Educational Ojective: Know the concept of confounding. Distinguish between crude and adjusted measures of association. Confounding refers to the bias that can result when the exposure-disease relationship is mixed with the effect of extraneous factors (i.e., confounders).

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