Lecture 05b - Association & Causation Role in...

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Role in Epidemiological Studies Aronson Feb 11 2010
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Outline Association Causation Conjecture and Refutation Assessing evidence Systematic reviews Meta-analysis
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Association or Causation?   Consider the story of a child who notices that the  rooster crows at sunrise every morning. The  child then concludes that the rooster causes the  sun to rise         Does the sunrise  cause  the rooster to crow? 
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Association • A   statistical relationship A measure of dependence between two  variables The degree to which the rate of a disease or  outcome is higher (or lower) among those with  an exposure as compared to those without an  exposure
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Association We’ve determine if an association exists  between an exposure and outcome (relative risk,  risk ratio, odds ratio or other correlation)  But is the association  real ? Types of association: 1. Spurious 2. Non-Causal 3. Causal
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Spurious Associations Two sources of spurious error 1)  Sampling error/bias Review: What do we mean when we  say  α  = 0.05? If 100 studies were completed, how many would we  expect to have incorrect findings?   Multiple comparisons within  a study also  problematic
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Spurious Associations 2)  Measurement Bias  = non-random error  introduced in the design, conduct or analysis of  a study Flawed measurement techniques Could result from measuring exposure differently  in those with disease/outcome compared to  those without E.g. electromagnetic radiation in leukemia cases  vs. non-cases
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Non-Causal Associations 1) May be due to confounding  factors = distortion due to mixing of exposures Example from Oleckno Cancer Gambling Smoking
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Non-Causal Associations 2) Also may be due to reverse causation  Important to establish the order of the exposure  and outcome Exposure must not be a result of the outcome A cross sectional study was completed measuring   current intake of vitamin C to see if high levels were  associated with reduced risk of stomach cancer     Vitamin C   Stomach Cancer Stomach Cancer   Vitamin C
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Lecture 05b - Association & Causation Role in...

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