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1、Causation in Epidemiology,Wang Yi School of Environmental Science and Public Health,WZMC Office:7A321 E-mail: ,Purpose of Epidemiology,To provide a basis for developing disease control and prevention measures for groups at risk. This translates into developing measures to prevent or control disease.
2、 Towards this purpose, epidemiology seeks to describe the frequency of disease and its distribution consider person, place, time factors assess determinants or possible causes of disease consider host, agent, environment,Causation- Example 1,What is “the cause” of myocardial infarction(心肌梗塞) ?,Concl
3、usion from example 1,Cause is the cumulative effects of multiple factors acting together (“causal interaction”). Causal factors almost never act alone. There are usually several mechanisms for disease causation.,Causation- Example 2,Most patients of lung cancer smoked. The proportion of smokers who
4、will develop lung cancer is much higher than the proportion of non-smokers who will develop the same disease. But still. Not all smokers will develop lung cancer. Some non-smokers will develop lung cancer.,Conclusion from example 2,Not all people exposed to an etiological factor will develop the dis
5、ease. Some people who are un-exposed to an etiological factor will also develop the disease.,What is causation？,Basic definition of “cause”: exposure that leads to new cases of disease; remove exposure and most cases do not occur. On a population basis: An increase in the level of a causal factor wi
6、ll be accompanied by an increase in the incidence of disease (all other things being equal). If the causal factor is eliminated or reduced, the frequency of disease will decline.,Historical Development of Theories of Disease Causation,Ancient times, illness occurred because of divine retribution for
7、 committing sins. 4th century BC, Hippocrates introduced idea of imbalance of four body humors (phlegm, yellow bile, blood and black bile) -but also hypothesized imbalances caused by changes in season, air, winds, water and stars, as well as personal habits. Mid 1800s, Pasteur, Berkeley and others i
8、ntroduced the germ theory of disease, that specific transmissible pathogens are responsible for disease.,Historical Development of Theories of Disease Causation,Mid to late 1800s, Henle and Koch developed postulates based on germ theory:,The agent must be shown to be present in every case of the dis
9、ease by isolation in pure culture. The agent must not be found in cases of other disease. Once isolated, the agent must be capable of reproducing the disease in experimental animals. The agent must be recovered from the experimental disease produced.,Historical Development of Theories of Disease Cau
10、sation,Henle and Koch did not consider these rigid criteria for causation Issue of specificity works better for infectious diseases than noninfectious diseases (“Aetiology: Kochs postulates fulfilled for SARS virus”, Nature 2004).,Epidemiological triangle,Historical Development of Theories of Diseas
11、e Causation,In 1960, concept of “web of causation” emerged in response to chronic diseases, which suggested that occurrence could be explained by many interconnected factors, including host and environment. Fundamental shift incorporating the idea of multiple causes of disease with the possibility o
12、f prevention at multiple steps, ie., that dont need a primary cause or to know the most important or direct of the causal factors.,Web of Causation,Web of Causation CHD(冠心病),Web of Causation,Historical Development of Theories of Disease Causation,In the 1950s/1960s, new set of criteria proposed, in
13、response to the process of the judgment of causation for smoking and lung cancer in the 1964 Surgeon Generals Report. Sir Austin Bradford Hills criteria for assessing causation: Strength of the association Consistency Specificity Temporality Dose response relationship Plausibility Coherence Experime
14、nt Analogy,Clinical observations,Available data (Ecological or Cross-sectional Studies),Case-control studies,Cohort studies,Randomized trials (only used for potentially beneficial treatments),Hypothesis Development,There are 4 different approaches can be used in the formulation of hypotheses（Mills C
15、annons） Method of Agreement Method of Difference Method of Concomitant Variation Method of Analogy,Mills Cannons,Method of Agreement If two or more instances of the phenomenon under investigation have only one circumstance in common, the circumstance in which alone all the instances agree, is the ca
16、use (or effect) of the given phenomenon. For example: Cigarette smoking caused an increased rate of lung cancer in males. Females followed the pattern and now have lung cancer rates higher than males.,Mills Cannons,Method of Difference If an instance in which the phenomenon under investigation occurs, and an instance in which it does not occur, have every circumstance in common save one, that one occurring only in the former, the circumstance in which alone the two instances differ is the
