patient registers.37 In total, around 9 million individuals were invited to participate. However, UK Biobank was only able to achieve a 5% response rate (∼500 000 participants recruited from ∼9 000 000 invited, personal communication, UK Biobank, 8 July 2016), and the resulting sample is not representative of the UK population as a whole. For example, the proportion of current smokers is relatively low in UK Biobank (19% in the general population vs 11% in UK Biobank, equivalent to an OR of 1.90),38 as is the proportion with no qualifications (25% vs 17%, equivalent to an OR of 1.63).39 Unsurprisingly, therefore, participants in UK Biobank have far lower rates of 5-year mortality than the UK population as a whole.40 Clearly, agreeing to take part in the UK Biobank study is associated with a number of characteristics that will reflect, for example, health status and social position. If non-smoking and having qualifications are both causally related to participation in UK Biobank, we would expect the association between smoking and having qualifications to be positively biased (i.e. we would expect to see a more positive association between genetic variants positively associated with smoking and whether participants had educational qualifications in UK Biobank
