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Association Causation. Marshall Tulloch-Reid Epidemiology Research Unit Tropical Medicine Research Institute. Objectives. Differentiate Association from Causation Understand the causal pie model for disease causation
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AssociationCausation Marshall Tulloch-Reid Epidemiology Research Unit Tropical Medicine Research Institute
Objectives • Differentiate Association from Causation • Understand the causal pie model for disease causation • List and discuss some of the criteria that have been used for deciding between association and causation – Henle Koch Postulates & the Bradford Hill Criteria • Understand the limitations of these criteria
Association Causation • Variables are associated if a statistical relationship exists between them • Statistical association does not imply a causal relationship between the variables
Association Causation r = 0.5
Association CausationHats and Obesity Percent Hats Percent Obese Group A B C 20 40 60 20 40 60
Concepts in Causation • The trigger of an event is often thought of as its cause • Most events are the result of the interaction of several causes • These causes do not need to occur simultaneously
Concepts in Causation A 88 year old women with uncontrolled type 2 diabetes and poor vision from diabetic retinopathy and glaucoma falls and fractures her hip. What caused the hip fracture?
The fall Poor vision Hypoglycemia Neuropathy Non-compliance with treatment Age Osteoporosis Genetic factors No non-skid rug Not wearing a hip protective device The dog Concepts in CausationWhat caused the hip fracture?
E D A C B The Causal Pie Single Component Cause Sufficient Cause
The Causal Pie SF II SF I SF – sufficient cause SF III
The Causal Pie • Sufficient cause • Component Cause • Necessary Cause
Causality and the Scientific Method Inductive method • Observations induce in the mind of the observer a suggestion for a more general statement • David Hume – inductive method depends on circular reasoning
Causality and the Scientific Method Refutation Karl Popper – statements about nature canbe corroborated by evidence but it does not amount to logical proof; however statements can be refuted by deductive logic
Criteria for Causation • A checklist is often employed to resolve issues of causation vs. association • Koch and Henle proposed criteria for infectious agents causing disease • The Bradford-Hill criteria is most often used for this purpose and can be more generally applied
Criteria for Causation First questions to be answered • Are the findings due to chance? • Are the findings due to bias? • Are the findings due to confounding?
Criteria for CausationHenle-Koch Postulates • The organism is always found with the disease • The organism is not found in any other disease • The organism, isolated from one who has the disease, and cultured through several generations, produces the disease (in experimental animals)
Temporal relationship Strength of the association Dose-response relationship Replication of findings Biologic plausibility Alternative explanations Cessation of exposure Specificity of the association Consistency with other knowledge Criteria for CausationBradford Hill Criteria
Criteria for Causation • Not all the criteria have to be meet for causality to be implied • Some criteria carry more weight than others
