Confounding and Simpson's paradox

doi:10.1136/bmj.310.6975.329b

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Multiple regression would confound the clinicians

C R Charig

Consultant urologist Epsom General Hospital, Epsom, Surrey KT18 7SG

EDITOR,—Steven A Julious and Mark A Mullee discuss confounding and Simpson's paradox, defining confounding as occurring “when the association between an exposure and an outcome is investigated but the exposure and outcome are strongly associated with a third variable. An extreme example of this is Simpson's paradox, in which this third factor reverses the effect first observed.”1 In their analysis of my and colleagues' …