Causation and Association
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Introduction
Associations are first identified, with causation being shown second. Associations are observed, while causation is inferred. Proving causality can be done with Hill's criteria.
Associations
Associations, or relationships, are statistical dependence between two or more events, characteristics, or other variables. However, it does not imply causation.
After designing a study to determine whether an association exists, work needs to be done to test what sort of relationship exists.
There are three different types of associations:
- Spurious: a false association due to chance or bias
- Non-causal: two factors of interest are both caused by another (potentially a confounder)
- Causal: change in the exposure leads to change in outcome
Measuring Associations
Associations can be measured in two ways.
Relative measures are presented as:
- ratios of exposed / unexposed
- relative risks and odds ratios
Absolute Measures are presented as:
- differences of exposed - unexposed
- attributable risk and number needed to treat (NNT)
Causation
Causes are complex and almost always act in concert with other factors that have various strengths and directions.
Indirect causes can include risk factors, but not all risk factors are causal.
Four types of causal factors
Necessary and sufficient
- without a given factor, disease does not develop
- ie HIV and AIDS
Necessary but not sufficient
- multiple factors, including the one of interest, required
- ie HPV infection and cervical cancer
Sufficient but not necessary
- factor can produce disease, but so can other independent factors
- ie radiation and benzene can cause leukemia, but neither requires the other
Neither sufficient nor necessary
- complex models of disease etiology
- ie high fat diet, hypertension, diabetes, etc all are related to heart disease
Proving Causality - Hill's Criteria for Causation
- temporal relationship: the only absolutely neccessary criterion
- strength of the association
- dose-response relationship
- consistency
- experiemental
- biolgicial plausibility
- coherence
- specificity
Resources and References
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