# Give an example of two variables for which you expect to be marginally associated but conditionally independent, controlling for $Z$.

Give a real world (qualitative) example of three variables $$X$$, $$Y$$ and $$Z$$, for which you expect $$X$$ and $$Y$$ to be marginally associated but conditionally independent, controlling for $$Z$$.

I can think of many example where correlation does not imply causation but I am having trouble thinking of examples for this.

• $X=Y=Z$ gives an example. – Kavi Rama Murthy Oct 4 '18 at 9:15