To the Editor Large cohort studies have transformed our ability to evaluate complex exposure-outcome relationships. However, these studies are typically not representative of the source population from which they are sampled because of selection at recruitment, attrition over time (in prospective studies), or both. For example, in UK Biobank, the response rate was approximately 5%, and the resulting sample was healthier and more highly educated than the general population of the United Kingdom.1
Munafò M, Smith GD. Biased Estimates in Mendelian Randomization Studies Conducted in Unrepresentative Samples. JAMA Cardiol. 2018;3(2):181. doi:10.1001/jamacardio.2017.4279
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