Fourth, the sampling used in many cohort studies is problematic for studying alcohol. Many cohort studies are restricted to established market economies, and cohorts are typically selected for ease of follow-up and the resulting high retention rate. As a result, many samples used for cohort studies are comprised of nurses, medical doctors, other health professionals, members of the American Cancer Society, or other middle to upper middle class professionals. While such cohorts may have enough variation in average volume of drinking for analyses, they are unlikely to have enough variation in terms of patterns of drinking and potential moderating lifestyle factors.
