A study was done to compare the lung capacity of farm workers to the lung capacity of administrative staff at a distribution facility. The researcher studied 200 workers of each type. Other factors that might affect lung capacity are smoking habits and exercise habits. The smoking habits of the two worker types were found to be similar, but the farm workers generally exercise more than the administrative staff.
1. Which of the following is a confounding variable in this study that the researcher should control for in the analysis?
a. Exercise
b. Lung capacity
c. Smoking status
d. Occupation