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Causality

Many factors can be associated with outcomes but few are meaningful causes.

In Epidemiology, the following criteria due to Bradford-Hill are used as evidence to support a causal association:

1. Plausibility (reasonable pathway to link outcome to exposure)
2. Consistency (same results if repeat in different time, place person)
3. Temporality (exposure precedes outcome)
4. Strength (with or without a dose response relationship)
5. Specificity (causal factor relates only to the outcome in question - not often)
6. Change in risk factor (i.e. incidence drops if risk factor removed)

Elwood's criteria are a modern extension of this concept:

1. Descriptive evidence
    exposure or intervention
    design
    population
    main result

2. Non-causal explanation
   chance
   bias
   confounding

3. Positive features
    time
    strength
    dose-response
    consistency
    specificity

4. Generalisability
    to eligible population
    to source population
    to other populations

5. Comparison with other evidence
    consistency
    specificity
    plausibility and coherence

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