Cohort studies: sources of bias

The association between adherence to lifestyle recommendations and risk of colorectal cancer was investigated using a prospective cohort study. Adherence to lifestyle recommendations was measured by a lifestyle index based on five factors—physical activity, waist circumference, smoking, alcohol intake, and diet.1

Between 1993 and 1997, all Danish men and women aged 50-64 years who lived in Copenhagen and Aarhus were invited to participate. In total, 160 725 potential participants were identified and 57 053 (35%) accepted the invitation. Of the respondents, 569 were excluded because they had recently been diagnosed with cancer. A further 997 respondents were excluded because of missing data, resulting in a cohort size of 55 487. Adherence to lifestyle recommendations was measured at recruitment. Cohort participants were followed …