Confounding. Objectives To define and discuss confounding To discuss methods of diagnosing...
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Transcript of Confounding. Objectives To define and discuss confounding To discuss methods of diagnosing...
Confounding
Objectives
• To define and discuss confounding
• To discuss methods of diagnosing confounding
• To define positive, negative and qualitative confounding
• To diagnose some data for confounding
Definition of Confounding
• A non-causal association between a given exposure and an outcome is observed as a result of the influence of a third variable (or group of variables) designated as confounding variable(s).
Rules of Confounding
• The confounding variable is:– Causally associated with the outcome– Non-causally or causally associated with
the exposure– Not an intermediate variable in the causal
pathway between exposure and outcome
Example• The association of alcohol related cirrhosis and TB
mortality• Which are predictors and confounders?
– Age– Race– Gender– SES– Homelessness– Nutrition– HIV status– Access to care– Resistant strain of TB– Adherence to TB medication
Predictor, Confounder or Outcome
• Cell phone, auto accident, young age• Vitamin A, diarrhea, childhood mortality• Hepatitis C, alcohol, mortality• Age, oral birth control, breast cancer
incidence• SES, race, cancer mortality• Gender, SES, heart attack attributable
mortality
Different strategies to assess confounding
• Examine crude and adjusted estimates of the association
• Stratification
Types of
• Positive – overestimation of the true strength of association
• Negative – underestimation of the true strength of association
• Qualitative – inverse in the direction of the association