Association and Causation

ASSOCIATION AND CAUSATION
Dr. Animesh Gupta
MD, FDM, FAGE
Associate Professor
Dept. of Community Medicine, NMCH, Jamuhar (Bihar)

Introduction
Association
■ Defined as the
concurrence of two
variables more often
than would be expected
by chance.
Causation
■ CAUSE- an event/
condition/ characteristic
which plays an important
role in occurrence of the
outcome (example:
smoking and lung cancer)
24/25/2020 Dr. Animesh Gupta Association & Causation

Concept of correlation
■ Correlation indicates the degree of association between two
characteristics.

Association - Types
Spurious
Association
Indirect
Association
Direct(causal)
Association

Spurious association
■ Association between a disease and suspected factor may
not be real.
■ For example – perinatal mortality higher in hospital births
than in home births.
5
PMR

Indirect association
■ Is a statistical association between a variable of interest
and a disease due to the presence of another factor.
■ This factor is common to both the variable and disease and
is known as the confounding factor (variable).

Indirect association….

Direct association
■ One-to-one causal relationship
■ Multifactorial causation

Direct association
One-to-one casual relationship
■ The variables are stated to be causally related (AB) If a
change in A is followed by a change in B.
■ When a disease is present, the factor must also be present.
Cause (A)
Effect (B)
4/25/2020 Dr. Animesh Gupta Association & Causation 9

Direct association…
■ Proponents of the germ theory of disease insisted that the cause
must be –
– Necessary
– sufficient for the occurrence of the disease
■ “Necessary and Sufficient” concept does not fit well for many
diseases, e.g; Tuberculosis
■ Tubercle bacilli is clearly a necessary factor, but its presence may
or may not be sufficient to produce the disease

Direct association…
■ A single factor may lead to more than one outcome.
■ Conclusion, one-to one causal relationship, although ideal in
disease aetiology but does not explain every situation.

Multifactorial causation
■ Non-communicable diseases have a different causal
thinking, the etiology is multifactorial or Web of Causation.
■ Eg: Both Asbestos exposure and Smoking cause Lung
Cancer independently.
■ Either the causes are acting independently or cumulatively .

Guidelines for Judging causality
■ Additional criteria was added by U.S SURGEON GENERAL
(1964), which was further strengthened by BRADFORD HILL
(1965) criteria.

ADDITIONAL CRITERIA FOR JUDGING
CAUSALITY
❖Bradford Hill and others- pointed out that the likelihood of a
causal relationship is increased by the presence of the
following criteria :
❖Temporal association
❖Strength of association
❖Specificity of the association
❖Consistency of the association
❖Biological plausibility
❖Coherence of the association4/25/2020 Dr. Animesh Gupta Association & Causation 14

1. Temporal association.
■ Does the suspected cause precede the observed effect?
■ The causal attribute must precede the disease or
unfavourable outcome.(Exposure before disease)
■ Length of interval between exposure and disease very
important.
■ Lung cancer occurs in smokers of long-standing, this
satisfies Temporal association

16

2. Strength of association.
■ Relative risk – is it large?
■ Is there dose-response, duration-response relationship?
■ With increasing level of exposure to the risk factor an increase
in incidence of the disease is found.
■ The causal relationship of cigarette smoking and lung cancer
has been based on 3 points:
– Relative risk
– Dose-response relationship
– The decrease in risk on cessation of smoking

3. Specificity of the association.
■ Specificity implies a one-to-one relationship between cause
and effect.
■ Establishment of specificity is a difficult criterion.
■ Cigarette smoking-associated-many diseases, reflecting lack
of specificity. But this cannot be an argument to dismiss the
causal hypothesis.
– Not everyone who smokes gets lung cancer
– Not everyone who developed lung cancer have smoked

4. Consistency of the association
■ Lack of consistency will weaken the causal relationship.
■ A consistent association has been found between smoking
and lung cancer.
■ 50 retrospective studies and 9 prospective studies in
different countries has shown a consistent association
between smoking and lung cancer.

5. Biological plausibility.
■ Causal association is supported if there is biological
credibility to the association, i.e, understanding of the
response of cells, tissues, organs and system to stimuli.
■ The criterion of biological plausibility should not be applied
rigidly.
■ Lung cancer-produced-tracheobronchial implantation of
tobacco extracts or by inhalation of cigarette smoke or of
aerosols of its constituents.

6. Coherence of the association.
■ The final criterion for causal significance of an association is
its coherence with the known facts that are thought to be
relevant.
■ Male and female differences in trends of lung cancer death
rates are also coherent with the more recent adoption of
smoking by women.

22

Conclusion
■ The Causal inferences resulted from the Epidemiological
Studies are very important to Public health and provide
inputs for Political and Judicial decisions.
■ Eg: The Causal association between Smoking and Lung
Cancer has resulted in labeling of Cigarette packets and
Increased campaign ads.
■ It is thus important for public health and policy makers to
understand the fundamentals of causal inference.

Association and Causation

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