This document discusses the differences between effect modification, bias, and confounding in epidemiological studies. It provides an example where age modifies the effect of sex on hospitalization after a motor vehicle collision. Effect modification exists when the effect of an exposure on an outcome differs across levels of another variable. This is not a bias, but rather an interaction that should be reported. Bias occurs when systematic errors distort the exposure-outcome association. Confounding happens when a third variable is associated with both the exposure and outcome but does not cause the outcome.

1. EFFECT MODIFICATION IN
EPIDEMIOLOGICAL STUDIES: A
BIAS OR AN INTERACTION?
OPEYEMI ABDULAHI
P21DLPH80148
COMM 818: ADVANCED EPIDEMIOLOGY

2. OUTLINE
• INTRODUCTION
• ILLUSTRATION
• EFFECT MODIFICATION VS BIAS
• EFFECT MODIFICATION VS CONFOUNDING
• CONCLUSION

3. INTRODUCTION
Exposure outcome
Effect modifier
• Effect modification happens when an extraneous variable modifies
the effect of the exposure of interest.

4. INTRODUCTION contd
• Consider exposure to asbestos and lung cancer
• Asbestos (no smoking) 3X risk of lung cancer
• Asbestos (plus smoking) 64X risk of lung cancer
• Smoking an effect modifier

5. ILLUSTRATION
• Consider the following hypothetical study comparing hospitalization
after a motor vehicle collision for male and female drivers.
Crude RR= 1.44
hospitalized Not hospitalized Total
Male 1330 7018 8348
female 798 6400 7198

6. ILLUSTRATION contd
Stratification by age
• Age< 40 years
Stratum-specific RR = 1.80
• Age > 40 years
Stratum-specific RR = 0.93
hospitalized Not hospitalized Total
Male 966 3146 4112
female 460 3000 3450
Hospitalized Not hospitalized Total
Male 364 3872 4236
Female 348 3400 3748

7. EFFECT MODIFICATION VS BIAS
• Bias = incorrect estimation of the true effect of the exposure and the
outcome due to systematic error in the design, recruitment, data
collection or analysis
• If the method used to select subjects or collect data results in an
incorrect association, THINK >> Bias!

8. EFFECT MODIFICATION VS CONFOUNDING
• Confounding = another variable distorting effect/association between
an exposure and outcome
• If an observed association is not correct because a different (lurking)
variable is associated with both the potential risk factor and the
outcome, but it is not a causal factor itself, THINK >> Confounding!

9. Bias
Interaction
(effect modification)

10. CONCLUSION
• Bias vs Confounding vs Effect Modification
• Effect modification is an interaction rather than a bias
• Effect modification is to be sought and reported

11. THANK YOU FOR YOUR ATTENTION!

12. REFERENCES
• Bovbjerg, M.L. (October, 2020) Foundations of Epidemiology.
Retrieved [August 7, 2022], from
https://open.oregonstate.education/epidemiology/
• Wayne, W. L & Sullivan, L. . Confounding and Effect Measure
Modification. Boston University School of Publich Health Retrieved
[August 7, 2022], from https://sphweb.bumc.bu.edu/otlt/mph-
modules/bs/bs704-ep713_confounding-em/bs704-
ep713_confounding-em_print.html