1. Hein Stigum http://folk.uio.no/heins/ courses
E8 DAGs intro 2h, Answers
Hein Stigum
courses
17. apr.
H.S.

2. Exercise: Physical activity and Coronary Heart Disease (CHD)
We want the total effect of Physical Activity on CHD. What would we adjust for?
Unconditional
Path
Type
Status 1
E®D
Causal
Open 2
E¬C1®D
Non-causal 3
E¬C2®D
Bias
Noncausal open=biasing path
Conditioning on C1 and C2
Path
Type
Status 1
E®D
Causal
Open 2
E¬[C1]®D
Non-causal
Closed 3
E¬[C2]®D
No bias
Apr-17
Apr-17
Apr-17
H.S.
H.S. 2 2

3. Tea and depression Total effect: adjust for O
Direct effect: adjust for C (and O)
Caffeine is both intermediate and part of a confounder path.
Path
Type
Status 1
E→D
Causal
Open 2
E→C→D 3
E←O→C→D
Non-causal
Tea and depression: Finnish study
Caffeine reduces depression: Nurses Health Study
direct
total
indirect
Apr-17
H.S.

4. Statin and CHD
We want the total effect of statin on CHD. What would you adjust for?
Can we estimate the direct effect of statin on CHD (not mediated through cholesterol)?
No adjustments gives
the total effect
Adjusting for C will close path 2
but will open path 3 and give bias!
C is a collider on path 3
Apr-17
H.S.

5. Survival bias Conclusion: Have survival bias
risk
Paths:
ED Causal Open
E[S]RD Non-causal Open E
early exposure D
later disease
E and R are unrelated causes for disease
Conclusion:
Have survival bias
Must adjust for R to remove the bias
17. apr.
H.S.

6. Exercise: causal pies Sufficient causes for Hospital: Selection bias:
Causal pies for hospital
Selection bias:
Sufficient causes for Hospital:
1) or
2) and
3) both
Selection bias:
Negative bias
Positive bias ?
Apr-17
H.S.

7. Exercise: Collider stratification
Hospital risk:
Selection bias
Collider stratification bias
Apr-17
H.S.