1. Epidemiology 503
Confounding

2. Confounding
A non-causal association between a given exposure and an outcome that is observed as a result of the influence of a third variable (confounder).

3. Example of Confounded Association

4. Example of Confounded Association

7. Confounding in a Causal Diagram
Health Status
Weight Death

8. Properties of a Confounder
Predictive of disease among the unexposed
Drinking Car Crashes
Associated with exposure in source population
Drinking Smoking
Not a downstream cause of the exposure (e.g., in the causal pathway from exposure to disease)
Smoking Drinking Car Crashes

9. Examples of No Confounding

10. Why is Confounding a Problem?
Because…...
The estimate of association between exposure and disease includes BOTH the contribution of the exposure AND the confounder
Confounding can overestimate true association (positive confounding) or underestimates true association (negative confounding)
Truth E
+++ D E + D E D ++ + - + + C C

11. Assessing Confounding in Your Data
CALCULATE the CRUDE measure of association between exposure and outcome (RR or OR)
ASSESS associations between a) exposure and confounder AND b) confounder and disease in non- exposed
CALCULATE the strata specific RRs or ORs
COMPARE crude to strata specific RRs or ORs
If measure of association is the same in the strata but different than the crude then you have evidence of confounding!

12. Step 1: Crude association between coffee drinking and cancer.
None
Coffee
165
735 85
1015
Total
250
1750
Unadjusted RR= 165/( ) / (85)/( ) =2.37
“Crude RR”

13. Step 2a: Is smoking associated with coffee drinking?
Cf+
Cf-
Total S+
800
200
1000 S-
100
900
1100
2000
PR for Coffee Drinking Among Smokers vs. Non-Smokers: (800/1000)/(100/1000) = 8

14. Step 2b: Is smoking associated cancer among non-coffee drinkers?
Total S+ 40
160
200 S- 45
855
900 95
1050
1100
RR for Cancer Among Smokers vs. Non-Smokers who don’t drink coffee: (40/200 )/ (45/900) = 4.0

15. Step 3: Is coffee associated with cancer within smoking strata?
In Smokers D+ D-
Total E+
160
640
800 E- 40
200
1000
In Non-Smokers D+ D-
Total E+ 5 95
100 E- 45
855
900 50
950
1000
RR Smoker Strata= (160/800)/(40/200)
= 1.0
RR Non-Smoker Strata= (5/100)/(45/900)
= 1.0

16. Step 4: Compare strata-specific RR to crude RR
Smoking confounds the association between coffee and cancer because:
The strata specific RR are equal (1.0) –
The strata specific RR of 1.0 is NOT the same as the crude RR of 2.37
The difference between the strata specific and crude is large so we conclude that this is important confounding
Some people use a 10% rule as a rough metric of “importance”

17. How to Report Data with Confounders
If you have a confounder:
DO NOT report crude OR or RR!
Instead report stratum-specific OR or RR or adjusted OR or RR from regression model
Other methods for summarizing strata-specific OR or RR (e.g., Mantel-Haenszel – a weighted average)