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Causation in Epidemiological Studies

Dr. Birgit Greiner

Senior Lecturer

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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

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Why bother?

Establishing causation is important for

• Effective treatment

• Health protection policies

• Introduction of environmental standards

• Disease prevention and education programmes

• Legal action

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“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

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Increased Susceptibility

Ingestion of Cholera vibrio

Cholera

Exposure to contaminated

water

Effect of cholera toxins on bowel

wall cells

Genetic Factors

Poverty

Malnutrition

Crowded housing

Risk factors for cholera Mechanisms for cholera

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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

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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

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ExampleYou 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?

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Observed association, could it be:

Assessing the Assessing the relationship relationship

between a possible between a possible cause and an cause and an

outcomeoutcome

Selection or measurement bias

Confounding

Chance

Causal

No

No

Probably Not

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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?

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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?

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Judging the evidence:

How many lines of evidence lead to the conclusion ?

Guidelines for causation cont.

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Average noise levelduring an 8-hour

working day(decibels)

Exposure time (years)

5 10 40

<80 0 0 0

85 1 3 10

90 4 10 21

95 7 17 29

100 12 29 41

105 18 42 54

110 26 55 62

115 36 71 64

Percentage of people with hearing loss

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Causal factor, causal link or mechanism?

SmokingArterio-sclerosis Heart disease

Stress SmokingArterio-

scleroris

Heart diease

Arterio-sclerosis

Heart disease