Endgames Statistical Question

Cohort studies: sources of bias

BMJ 2011; 343 doi: http://dx.doi.org/10.1136/bmj.d7839 (Published 07 December 2011) Cite this as: BMJ 2011;343:d7839
  1. Philip Sedgwick, senior lecturer in medical statistics
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  1. p.sedgwick{at}sgul.ac.uk

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 …

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