Causation in Epidemiological Studies

1. Causation in Epidemiological Studies Dr. Birgit Greiner Senior Lecturer

2. Learning objectives • Understand the definition of a causal factor for health and illness • Differentiate between causal factors and mechanisms for disease • Learn criteria how to practically determine causality • Be able to apply criteria for causality to an example

3. Why bother? Establishing causation is important for • Effective treatment • Health protection policies • Introduction of environmental standards • Disease prevention and education programmes • Legal action

4. A Cause “An event, condition or characteristic that preceded the disease event and without which the disease event either would not have occurred at all or would not have occurred until some later time.” Rothman, 1998

5. Causes of Cholera Effect of cholera toxins on bowel wall cells Exposure to contaminated water Genetic Factors Poverty Ingestion of Cholera vibrio Increased Susceptibility Cholera Malnutrition Crowded housing Risk factors for cholera Mechanisms for cholera

6. Factors in causation • Predisposing: age, gender and previous illness create a state of high susceptibility • Enabling: low income, poor nutrition, poor medical care favour development of disease Precipitating: exposure to noxious agent may be associated with onset of disease • Reinforcing: repeated exposure to stress might aggrevate established disease

7. The web of causation Sufficient cause • If cause is present disease always occurs Necessary cause • If cause is absent the disease cannot occur Component cause • Component of a sufficient cause

8. Example You are presented with evidence from a case-control study showing that women hospitalised for breast cancer are more likely to have a family history of breast cancer as compared to women hospitalised for other reasons. From this study can you tell that family history of breast cancer is a cause for breast cancer?

9. Observed association, could it be: Selection or measurement bias No Assessing the relationship between a possible cause and an outcome Confounding No Chance Probably Not Causal

10. Guidelines for causation • Temporal relation:Does the cause preceed the effect? • Plausability:Is the association supported by other knowledge (mechansims of action, theory, experiments?) • Consistency:Have other studies shown similar results?

11. Guidelines for causation cont. • Strength:Is the strength of the association between the cause and the effect consistently high? • Dose-response relationsship: Is increased exposure to the cause associated with increased effect? • Reversibility: Does the removal of a potential cause lead to reduction in the effect? • Study design: Is the evidence based on a strong study design?

12. Guidelines for causationcont. Judging the evidence: How many lines of evidence lead to the conclusion ?

13. Dose-Response Relationship Percentage of people with hearing loss

15. Causal factor, causal link or mechanism? Arterio-sclerosis Heart disease Arterio-sclerosis Smoking Heart disease Heart diease Arterio- scleroris Stress Smoking