4.2.2. confounding classical approach
- 1. Second approach: “Classical” approach based on a priori criteria 2014 Page 1 18 “Bias of the estimated effect of an exposure on an outcome due to the presence of a common cause of the exposure and the outcome” – Porta 2008 ● A factor is a confounder if 3 criteria are met: ● a) a confounder must be causally or noncausally associated with the exposure in the source population (study base) being studied; ● b) a confounder must be a causal risk factor (or a surrogate measure of a cause) for the disease in the unexposed cohort; and ● c) a confounder must not be an intermediate cause (in other words, a confounder must not be an intermediate step in the causal pathway between the exposure and the disease)
- 2. 19 Exposure E 2014 Page 2 Disease (outcome) D Confounder C Confounding Schematic Szklo M, Nieto JF. Epidemiology: Beyond the basics. Aspen Publishers, Inc., 2000. Gordis L. Epidemiology. Philadelphia: WB Saunders, 4th Edition.
- 4. Exposure E Confounder C General idea: a confounder could be a ‘parent’ of the exposure, but should not be be a ‘daughter’ of the exposure Disease D 21 2014 Page 4
- 5. Example of schematic (from Gordis) 22 2014 Page 5
- 6. Birth Order E 2014 Page 6 23 Down Syndrome D Confounding factor: Maternal Age C Confounding Schematic
- 7. HRT use Heart disease Association between HRT and heart disease Confounding factor: SES 2014 Page 7 24 Are confounding criteria met?
- 8. BRCA1 gene Breast cancer Confounding factor: Age x 2014 Page 30 25 Are confounding criteria met? Should we adjust for age, when evaluating the association between a genetic factor and risk of breast cancer? No!
- 9. Sex with multiple partners Cervical cancer Confounding factor: HPV Are confounding criteria met? 2014 Page 9 26
- 10. Sex with multiple partners HPV Cervical cancer 2014 Page 10 27 What if this was the underlying causal mechanism?
- 11. Obesity Mortality Are confounding criteria met? 2014 Page 11 Confounding factor: Hypertension 28
- 12. Obesity Hypertension Mortality 2014 Page 12 29 What if this was the underlying causal mechanism?
- 13. Direct vs indirect effects Obesity Hypertension Mortality Obesity Indirect effect Hypertension Mortality Direct effect 2014 Page 13 Direct effect is portion of the total effect that does not act via an intermediate cause 30 Indirect effect
- 14. Hernan MA, et al. Causal knowledge as a prerequisite for confounding evaluation: an appl3ic3ation to birth defects epidemiology. Am J Epidemiol 2002;155(2):176-84. 2014 Page 14 Simple causal graphs DC E Maternal age (C) can confound the association between multivitamin use (E) and the risk of certain birth defects (D)
- 15. 34 Complex causal graphs Hernan MA, et al. Causal knowledge as a prerequisite for confounding evaluation: an application to birth defects epidemiology. Am J Epidemiol 2002;155(2):176-84. E DC U History of birth defects (C) may increase the chance of periconceptional vitamin intake (E). A genetic factor (U) could have been the cause of previous birth defects in the family, and could again cause birth defects in the current pregnancy 2014 Page 15
- 16. 35 Smoking A E Calcium D Bone fractures C BMI supplementation U Physical Activity B 2014 Page 16 Source: Hertz-Picciotto More complicated causal graphs!
- 17. The ultimate complex causal graph! 36 A PowerPoint diagram meant to portray the complexity of American strategy in Afghanistan! 2014 Page 17