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. Author manuscript; available in PMC: 2014 Dec 9.
Published in final edited form as: Nat Rev Rheumatol. 2014 Apr 1;10(7):403–412. doi: 10.1038/nrrheum.2014.36

Figure 2.

Figure 2

A causal diagram of a typical observational study showing the assessment of the effect of obesity on OA progression among patients with (incident) OA. Conditioning on (or restricting to) those with OA incidence (i.e. conditioning on a common effect, as explained in Figure 1) results in obesity and the URFs becoming negatively associated, as indicated by a dotted line between obesity and URFs, even though these two factors were not associated before OA incidence. This artificially-generated negative confounding results in a biased association between obesity and OA progression (represented as obesity—URF→OA progression), leading to effect estimation biased towards the null (see Figure 1 legend for details). Abbreviations: OA, osteoarthritis; URFs, unknown or unmeasured risk factors.