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Definition of Confounding

EPID 623-88 Introduction to Analysis and Interpretation of HIV/STD Data Confounding Manya Magnus, Ph.D. Summer 2001 adapted from M. O’Brien and P. Kissinger. Definition of Confounding.

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Definition of Confounding

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  1. EPID 623-88Introduction to Analysis and Interpretation of HIV/STD DataConfoundingManya Magnus, Ph.D.Summer 2001adapted from M. O’Brien and P. Kissinger

  2. 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).

  3. 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

  4. 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

  5. Different strategies to assess confounding • Examine crude and adjusted estimates of the association • Stratification and examination of measures of association by strata

  6. Crude Associations

  7. More ideas about confounding • Partial confounding can occur (not an all or nothing thing) • Residual confounding (occurs when categories of confounders controlled for are too broad or when confounding variables remain unaccounted for)

  8. Collinearity

  9. Effect Modifiers

  10. Interaction • Two or more risk factors modify the effect of each other with regard to the occurrence or level of a given outcome • Also known as effect modification • Synergistic (positive interaction) – potentiates the effect of the exposure of interest • Antagonistic (negative interaction) – diminishes or eliminates the effect of the exposure of interest

  11. Confounding versus Interaction • Sometimes the same variable may be both a confounder and an effect modifier • Confounding makes it difficult to evaluate whether a statistical association is also causal • Interaction is part of the web of causation • Do not adjusted for a variable that is both a confounder and an effect modifer (reporting an average odds may be meaningless)

  12. Risk factors for sinusitis among HIV-infected persons in Multivariate logistic regression

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