Confounding, Effect Modification, and Stratification

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Confounding, Effect Modification, and
Stratification

• Yu-Fen Li

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Adding a Third Dimension to the RxC contingency
tables
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Confounding

• A confounding variable is associated with the
  exposure and it affects the outcome, but it is
  not an intermediate link in the chain of
  causation between exposure and outcome.

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Confounding example
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Confounding example
Smokers
Non-smokers
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Confounding example
Smoking
X
Drinking
Lung cancer

• Drinking is not associated with lung caner
• Smoking is a confounder

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Controlling for confounders

• Confounders can be controlled for in the design
  phase of a study (randomization/
  restriction/matching).
• Confounders can be controlled for in the analysis
  phase of a study (stratification/ multivariate
  model).

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Analytical identification of confounders

• Stratification
• Mantel-Haenszel Procedure
• Take a 2x2 table and break it into a series of
  smaller 2x2 tables (one table at each of G levels
  of the confounder yields G tables)
• Homogeneity test/ Mantel-Haenszel Summary Odds
  Ratio
• Example in testing for an association between
  lung cancer and alcohol drinking (yes/no),
  separate smokers and non-smokers.

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Mantel-Haenszel Procedure Pros and Cons

• Advantages
• Mantel-Haenszel summary statistic is easy to
  interpret and calculate
• Gives you a hands-on feel for the data
• Disadvantages
• Requires categorical confounders or continuous
  confounders that have been divided into intervals
  
• Cumbersome if more than a single confounder

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Effect modification

• Effect modification occurs when the effect of an
  exposure is different among different subgroups.
  
• aka interaction

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Interaction example
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Interaction example
Smokers
Non-smokers
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Interaction example
Smoking
Drinking
Lung cancer

• Drinking is associated with lung caner
• Smoking is an effect modifier

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Investigating interaction

• Effect modifiers can NOT be controlled
• Effect modification is not a bias, but a
  phenomena
• Effect modifiers can be evaluated in the analysis
  phase of a study (stratification/ multivariate
  model with an interaction term).

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HOMEWORK Q1
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HOMEWORK Q2
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HOMEWORK Q2