Association

&
Causation

Dr. Aliya Junaid

 Contents
 Association
 Types of Associations
 Causal Association Criteria
 MCQs
Epidemiology
 Epidemiology
 Epidemiology is the study of
distribution and determinants of
disease in population and application
of this study to control health problems
 Association & Causation
 Descriptive studies help in identification of
disease problem in community by;

relating disease to host, agent &
environmental factors
 & suggest an etiological hypothesis.

 Analytical & experimental studies confirm or

refute the observed association between
suspected causes and disease.
Association
 Association
 It is defined as Concurrence of two
variable more often than would be
expected by chance.

 In other words events are said to be

associated when they occur more
frequently together than one would
expect by chance.
 Association can be broadly grouped
under 3 headings

a) Spurious Association
b) Indirect Association
c) Direct (causal) Association
i. One-to-one Causal Association
ii. Multi factorial Causation
Spurious Association
 Spurious Association
 Sometime an observed association
between a disease and suspected factor
may not be real.
 E.g.
 A study was conducted between births at home and
births in hospital.
 Apparently Perinatal Mortality was higher in
hospital births than in home birth.
 It may be concluded that home deliveries are safer
than hospital deliveries.
 Such a conclusion is spurious because in general,
hospitals attract women at high risk for delivery
because of their special equipment and expertise.
Indirect Association
 Indirect Association
 Many associations which at first
appeared to be causal have been found
on further study to be due to indirect
association.
 Indirect association is a statistical association
between a characteristic of interest and a
disease due to presence of a third common
(confounding) factor, known or unknown.

 Such confounding variables ( age, sex, social

class) are potentially present in all data and
represent an obstacle to overcome in trying to
assess the causal nature of the relationship.
 E.g
 Altitude and endemic Goiter
 Altitude and
Endemic Goiter
B
Endemic
Goiter
C
Iodine
Deficiency
A
Altitude
 Sucrose and
Chronic Heart Disease
B
Sucrose

C
Cigarette
Smoking
A
CHD
Direct Association
 Direct Association
 One-to-one causal relationship
 Multifactorial Causation
One-to-One Causal
Relationship
One-to-one Causal Relationship
 Two variable are stated to be causally
related (AB) if a change in A is followed
by a change in B.
 This is known as one-to-one causal
relationship.
 Suggesting that when the factor A is
present, the disease B must result.
 E.g Measles
One-to-One Causal
Relationship

 RNA Paramyxovirus

Measles
 Multi Factorial Causation
 Especially important in non-
communicable disease i.e CHD, Lung
Cancer
 Multi Factorial Causation
 Each of the factor below can act
independently to produce a disease e.g
air pollution, smoking , asbestos
exposure Lung cancer
Factor 1
Reaction At Disease
Factor 2 Cellular Level

Factor 3
 Multi Factorial Causation
 Each factor can cumulatively produce a
disease and can have a synergistic effect.

Factor 1
+
Factor 2 Reaction At
Disease
+ Cellular Level
Factor 3
Causal Association
 Causal Association
 In the absence of controlled
experimental evidence to incriminate
the cause, certain additional criteria
have been evolved for deciding when
an association maybe considered a
causal association
 Causal Association
The following concepts are used by
epidemiologists in making a causal
inference:

a) Strength of association
b) Consistency of the observed
association
c) Specificity of the association
d) Temporal sequence of events
e) Dose-response relationship
f) Biological plausibility of the
observed association
g) Coherence of the association
h) Experimental evidence
i) Reversibility
Strength of Association
 Strength of Association
 The strength of association is measured by;
 Relative risk-------is it large?
 Ratio of disease incidence among exposed and

non-exposed
 E.g. RR of lung cancer = 10: 1

 Is there a dose-response, duration-response

relationship?

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 Large the relative risk, greater the
likelihood of a causal association.
 Causal relationship is strengthened if there
is a biological gradient or dose-response
relation.
 Increasing exposure to risk factors increase
rise in disease incidence.
 Consistency of the
Observed Association
 Consistency of the
Observed Association
 Confirmation by repeated findings of an
association in case-control and cohort studies
in different population groups and different
settings strengthens the inference of a causal
connection.
 Many cohort and case-control studies have
shown an increased risk of cardiovascular
disease associated with oral contraceptive use.
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Specificity of the Association
Specificity of the Association

 It was formerly thought that to be causal, a

one to one relationship should exist between
the exposure and the disease;
 one exposure should cause one disease, and no
other exposures should cause the disease.
 This has its roots in the bacteriological model
where one microorganism is associated with
one disease.
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 Specificity of a relationship between
exposure and outcome strengthens
confidence in a causal inference, but
lack of specificity does not rule out
causality.
Temporal sequence of events
 Temporal sequence of events
 It seems obvious that in order for an
exposure to cause an event (disease), it must
precede and not follow the disease.

 One example is the study of prenatal

exposure and malformations.

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Dose-response relationship
 Dose-response relationship
 If a factor is of causal importance in the
occurrence of a disease, then the risk of
developing the disease should be related to
the degree of exposure to the factor, i.e., a
dose-response relationship should exist.

 The dose-response relationship between

serum cholesterol level and the risk of
coronary heart disease is an example.
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Biological Plausibility of the
Observed Association
 Biological Plausibility of the
Observed Association
 A causal hypothesis must be viewed in the
light of its biological plausibility.
 That is, association agrees with current
understanding of the response of cells,
tissues, organs and system to stimuli.

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 Cigarette smoking and lung cancer hypothesis is
biological plausible.

 Its not hard to visualize the inhalation of hot smoke

into lungs and deposition of a chemical carcinogen
over a period of long time will initiate neoplastic
changes in the lungs.

 Positive association of food intake and skin cancer

makes no biological sense.
Coherence of the association
 Coherence of the association
 The association is in line with the known
facts that are thought to be relevant.
 For example the historical evidence of the
rising consumption of tobacco in the form of
cigarettes and the rising incidence of lung
cancer are coherent.

46
Experimental evidence
 Experimental evidence
 The randomized clinical trial (RCT) is the
closest approximation in epidemiology to an
experiment, and a well-run trial may
confirm a causal relationship between an
exposure and an outcome.

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Reversibility
 Reversibility
 When the removal of a possible cause results
in a reduced disease risk, the likelihood of
the association being causal is strengthened.

 For example the cessation of cigarette

smoking is associated with a reduction in the
risk of lung cancer relative to that in people
who continue to smoke.

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 Observed Association
Could it be due to selection or
Measurement bias?

NO

Could it be due to confounding?

NO

Could it be a result of chance?

PROBABLY NOT

Could it be causal?

Apply guidelines and make judgment

MCQs
 A researcher concluded a cohort study
with a relative risk of 8.5 and proved
an association between a given food
and skin cancer. The most apparent
reason in this particular case for
rejection of association is:
a) Specificity
b) Consistency
c) Coherence
d) Strength of Association
e) Biological Plausibility
e. Biological plausibility

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 If the association between two variables
given by a researcher is simulated by
other researchers too then it has:

a) Specificity
b) Consistency

c) Coherence

d) Temporal Sequence

e) Biological Plausibility
b. Consistency

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 If a virus does not results in any other
disease other than chickenpox, this
association exhibits:

a) Specificity
b) Strength of Association
c) Coherence
d) Temporal Sequence
a. Specificity

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Any Questions ?
Thank you