In 2005, Ross and colleagues found a protective association between maternal multivitamin supplementation during the periconceptual period and acute lymphoblastic leukaemia among children with Down’s syndrome (OR 0.51; CI 0.30 to 0.89).1 In their discussion they noted, “Maternal vitamin supplementation was collected by self-report, which is subject to…recall bias. However, a validation study…reported excellent agreement...” The validation study appears to imply that the bias from misclassification should be minimal. But given the number of false-positive published research findings,2 which are partly explained by issues of misclassification, the discussion of the potential misclassification raises more questions than it answers.

First, we want to know what is the expected magnitude and direction of the bias. Second, given the potential bias, how we should interpret the 95% CI which is calculated assuming no bias?3 And how should we as readers integrate ideas about the magnitude and direction of the bias into our interpretation of the results. For example, if we believe the misclassification …