3. Introduction
Association may be defined as the concurrence
of two variables more often than would be
expected by chance.
Epidemiological studies determine various
associations between an exposure and a disease.
Further, its important to find out whether the
exposure is causal for the disease or not.
4. Types of association
•Spurious association
•Indirect association
•Direct (causal) association
a) One-to-one causal association
b) Multifactorial association
5. Spurious association
Association between a disease and suspected
factor may not be real.
Indirect association
Associations which at first appeared to be
causal have been found on further study to be due
to indirect association. This is due to a
confounding variable.
6. Example for Spurious association:
PMR of 5.4 PMR of 27.8
Example for Indirect association:
High altitude Goiter
7. Direct (Causal) association
a)One-to-One causal association
Two variables(AB) are stated to be
causally related if a change in A is
followed by a change in B.
b)Multifactorial association
Considered when the etiology is
multifactorial.
All the causal factors can act
individually or cumulatively to produce
the outcome.
8. One to one causal association-
Measles virus Measles
Multifactorial association-
CHD
9. Criteria for Causal Association
Bradford Hill’s criteria for making causal inferences-
1.Strength of association
2.Dose-Response relationship
3.Lack of temporal ambiguity
4.Consistency of findings
5.Biologic plausibility
6.Coherence of evidence
7.Specificity of association
10. 1.Strength of association
• Measured by the relative risk (or odds
ratio).
• The stronger the association, the more
likely it is that the relation is causal.
• Relative risk is the ratio of the incidence
of the disease among exposed and the
incidence among non-exposed.
12. 2.Dose-Response relationship
• As the dose of exposure increases, the risk
of disease also increases
• If present, it is strong evidence for a causal
relationship.
• Absence of a dose-response relationship
does not necessarily rule out a causal
relationship.
• In some cases in which a threshold may
exist, no disease may develop up to a
certain level.
14. 3.Lack of temporal ambiguity
• Exposure to the factor must have occurred
before the disease developed
• The temporal relationship is important in
regard to the length of the interval
between exposure and disease
• It’s easier to establish a temporal
relationship in a prospective cohort study
than in a case-control study or a
retrospective cohort study.
16. 4.Consistency of findings
• The relationship should
be found consistently in
different studies and in
different populations.
• Unless there is a clear
reason to expect different
results, replication of the
findings should be there.
17. 5.Biologic plausibility
• Biologic plausibility refers to coherence with
the current body of biologic knowledge
• Epidemiologic findings should be consistent
with existing biologic knowledge.
• Example- Carcinogens from
cigarette smoke deposits in the
lung over a period of time
leading to lung cancer.
18. 6.Coherence of evidence
• If a relationship is causal, we would
expect the findings to be consistent with
other data.
• For the appraisal of causal significance of
an association it should be coherent with
known facts that are thought to be
relevant.
19. Example- Peptic ulcer disease
• Prevalence of H.pylori is same
in men as in women. Incidence of
duodenal ulcer in both have been
proved to be equal in recent
years.
• Prevalence of peptic ulcer
disease is believed to have
peaked in the latter part of 19th
century cause of poor living
standards.
20. 7.Specificity of association
• Association is specific when a certain
exposure is associated with only one disease
• When specificity of an association is found,
it provides additional support for a causal
inference
• Absence of specificity in no way negates a
causal relationship.
21. Example-
Prevalence of H.pylori in
patients with duodenal ulcer
is 90% to 100%.
However, it is found even in
some patients of gastric
ulcer and even in
asymptomatic individuals.
22. Few other criteria which might be
useful are:
• Cessation of exposure- Risk of the
disease declines when exposure to the
factor is reduced or eliminated.
• Consideration of alternate explanations-
Extent to which the investigators have
taken other possible explanations into
account and the extent to which they have
ruled out such explanations are important
considerations.
24. References:
• Gordis L. Epidemiology. 4th
ed.
Saunders Elsevier : Philadelphia ;
2009. Pg 227 to 246.
• Park K.Textbook of Preventive and
Social Medicine. 21th
ed.
Bhanarasidas Bhanot : Jabalpur
(India); 2011. Pg 84 to 87.
