Genetic Factors

1
Observational Study

• 
  
• 
  Genetic Factors
• Obesity
  
  
  
  

ROA progression
Preexisting ROA (K/L 2, 3)
If obesity or any other risk factors could not
cause knees to have K/L grade 3 or 4 ROA before
they first cause K/L grade 2 or 3 ROA, then
preexisting ROA (i.e., K/L 2, 3) will be an
intermediate step on the causal pathway between
obesity and ROA progression. Conditioning on
preexisting ROA will completely eliminate the
effect of obesity on risk of ROA progression.
Provided by Yuqing Zhang, DSc
2
Observational Study

• 
  
• 
  Genetic Factors
• Obesity
  
  
  
  

ROA progression
Preexisting ROA (K/L 2, 3)
Conditioning on baseline ROA results in its
causes (i.e., obesity and the genetic factor)
becoming directly associated, as indicated by a
dotted line between obesity and genetic factors,
even though these two factors are not associated
before the knees developed ROA. Such conditioning
opens an alternative path from obesity to ROA
progression (i.e., obesity --- genetic factor ?
ROA progression), thus biasing the effect of
obesity on ROA progression.
Provided by Yuqing Zhang, DSc