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Brief Look at Association vs causation


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A very brief summary of causation vs. association, causal pathways, the Bradford Hill Criteria, and Pyramid of Clinical Evidence.

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Brief Look at Association vs causation

  1. 1. Association vs. Causation Patrick B. Barlow PhD Candidate in Evaluation, Statistics, & Measurement The University of Tennessee This and many other lecture materials are available at
  2. 2. In this Presentation… Section I: Association vs. Causation, what does it look like?  Conceptually speaking  Casual Pathways Section II: Methods for judging strength of causal relationship  Bradford Hill Criteria  Pyramid of Clinical Evidence
  3. 3. Some Terms Association Causation Causal Pathway Interaction / Effect Modification Confounding / Mediation
  4. 4. Association vs. Causation Conceptually Speaking Association  Correlation  Regression  Odds / Relative Risk Causation  Influence  Effect  Confounding  Explanation  Dependence  Intervention  Likelihood  Randomization  Conditional  Attribution Adapted From: Joffe et al. Emerging Themes in Epidemiology 2012
  5. 5. Association vs. Causation Conceptually Speaking Association  Two observed variables that are jointly distributed  Can be strong, weak, positive, or negative. Causation  “A causal relationship is one that has a mechanism that by its operation makes a difference” (Joffe et al., 2012).  For example:
  6. 6. Causal Pathways When in doubt, draw it out! Causal Pathways  Visual depictions of the proposed links between a given exposure(s) and the outcome of interest. They help researchers understand the mechanisms at work in a complex disease process. Exposure ? Outcome
  7. 7. Types of Causal Pathways Simple (Direct) Confounding (Mediation) Effect Modification (Moderation) Exposure Exposure Exposure ? ? Outcome Confounder ? Outcome Effect Moderator ? Outcome
  8. 8. Types of Causal Pathways Simple (Direct) Confounding (Mediation) Effect Modification (Moderation) Diet Diet Diet ? ? CAD Smoking Hx ? CAD Activity Level ? CAD Examples adapted from
  9. 9. Methods for Judging Strength of Causal Relationship Bradford Hill Criteria Pyramid of Clinical Evidence
  10. 10. The Bradford Hill Criteria  Provides researchers with seven criteria for assessing strength of evidence.  Strength of association (i.e. effect size)  Consistency (i.e. reliability)  Specificity  Temporal relationship  Biological gradient  Plausibility  Coherence  Experiment (reversibility)  Analogy (consideration of alternate explanations)
  11. 11. The Bradford Hill Criteria a closer look…  Strength of association (i.e. effect size)  Is the magnitude of the association both statistically and clinically meaningful?  Consistency (i.e. reliability)  Can these results be reproduced? Do they coincide with existing research on the same phenomenon?  Specificity  Association is specific when the exposure is associated with only one disease  Temporal relationship  Does the exposure (cause) precede the outcome (effect)?
  12. 12. The Bradford Hill Criteria a closer look…  Biological gradient  Dose-response relationship can be established  Biological Plausibility  Is the association coherent with the current body of biologic knowledge?  Coherence  Is the association plausible given what we already know about the disease or disorder?  Experimental evidence  Has this association been shown in a randomized experimental trial?  Analogy (consideration of alternate explanations)  Have all other plausible alternative explanations been considered and ruled out? “The third variable issue”
  13. 13. Strength of Evidence & Study Design Which criteria are difficult to satisfy with…  Case – Control studies?  Cross – Sectional studies?  Cohort – Studies?
  14. 14. Pyramid of Clinical Evidence Systematic Reviews & Meta-analyses Cross-Sectional Studies: Level “2.3” Evidence Summaries RCT Cohort Studies Level 1 Evidence Level 2 Evidence Case Control Studies Level 3 Evidence Case Series Case Reports Ideas, Editorials, Opinions Animal research In vitro (‘test tube’) research
  15. 15. So, when reading an article… Whenever digging into a new controversy or research area, it is helpful to use methods for judging the strength of the relationship being discussed.  What type of study design are the using? (Pyramid of evidence)  How strong is the evidence? (Bradford Hill Criteria)  What is the clinical vs. statistical significance being presented?