In its simplest form confounding occurs when a third variable influences the association between an exposure and outcome. A variable is classified as a confounder when it is related to the exposure and the outcome and it is NOT an intermediate factor in the pathway between the exposure and outcome. A total of 17,968 individuals living in County X, who were between the ages of 45 and 65 years in 1973, were enrolled in a prospective study. Cases were identified during a 12-year follow-up period. By 1985, 160 cohort enrollees had been diagnosed with colon cancer. The control group consisted of a random sample of the total cohort at baseline. A total of 160 controls were selected. In this exercise, students will assess whether years of education confounds the association between an exposure cigarette smoking and the outcome colon cancer. Part I Question 1 - What type of study design was used by the investigators