Confounding in Epidemiology

1. Dr Athar Khan
Associate Professor
Department of Community Medicine
Liaquat college of Medicine & Dentistry
Email: matharm@yahoo.com

8. Esophageal Cancer Risk Factors
 Age
 Gender
 Gastroesophageal reflux disease
 Barrett’s esophagus
 Tobacco and alcohol
 Obesity
 Diet
 Achalasia
 Plummer-Vinson syndrome
Esophageal Cancer Risk Factors
 Workplace exposure
 Injury to the esophagus
 Tylosis
 History of certain other
cancers
 Human papillomavirus (HPV)
infection
Causes ??

11. 1. Criteria for confounding
A confounder is a variable that distorts the association between
two other variables (the exposure and the outcome). Often the
exposure is what is being studied as a potential cause of the
outcome, such as tooth loss
For a variable to be a confounder, it must have three
characteristics:
1) The confounding factor must be associated with both the risk
factor of interest and the outcome.
2) The confounding factor must be distributed unequally
among the groups being compared.
3) A confounder cannot be an intermediary step in the causal
pathway from the exposure of interest to the outcome of
interest

12. For a variable to be a confounder, it must have three
characteristics:
1) The confounding factor must be associated with both the risk
factor of interest and the outcome.
C
DE
Age is a confounding factor because it is associated with the exposure
(meaning that older people are more likely to be inactive) but is not a
results of E, and it is also associated with the outcome (because older
people are at greater risk of developing heart disease).

13. For a variable to be a confounder, it must have three
characteristics:
2) The confounding factor must be distributed unequally
among the groups being compared.
If the age distribution is similar in the exposure groups being
compared, then age will not cause confounding.

14. For a variable to be a confounder, it must have three
characteristics:
3) A confounder cannot be an intermediary step in the causal
pathway from the exposure of interest to the outcome of
interest
Smoking is NOT
a consequence of
coffee drinking
HDL is a
consequence of
alcohol consumption

18. 2. Types of confounders
Confounders can be classified into two categories:
Quantitative:
After adjusting for qualitative confounders, the association
between exposure and outcome completely disappears or
even reverses direction, meaning that the quality or nature
of the association changes.
In examples 2 and 3, the association disappeared after
adjusting for age, which was the confounder in both cases.
In example 4 for a confounder that reverses the direction of
association.

21. Unlike that for qualitative confounders, adjusting for
quantitative confounders only changes the magnitude of the
association but not its nature.

22. Unlike that for qualitative confounders, adjusting for
quantitative confounders only changes the magnitude of the
association but not its nature.

30. Simpson’s Paradox

31. Simpson’s Paradox

36. Odds Ratio
2.40
Odds Ratio 1.6 after
adjusting UF

47. In a study of the association of L-tryptophan with eosinophilia-myalgia
syndrome, confounding by indication may occur because L-tryptophan is
indicated to treat insomnia and depression, two disorders commonly
associated with myalgias, particularly eosinophilia-myalgia syndrome

48. Confounding by indication