I am most certainly not a statistician but what I think they are saying is that the composition of the microbiota and how a baby is fed are related (actually we can say that one causes the other) and so if you adjust for how the baby was fed it will not be possible to see what impact the microbiota has on its own (ie "inference towards the null").
Karleen Gribble
Australia

> 
> can a statistician among us, explain this to me???  Thanks, Pat in SNJ
> 
> from a current study on gut microbiota and bronchiolitis in Pediatrics
> 
> "We did not control for breastfeeding status because it was considered an ancestor variable of the association of interest (ie, the relationship between breastfeeding and likelihood of bronchiolitis may be mediated by gut microbiota), and adjustment of an ancestor variable would bias the inference toward the null."

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