Principles of Epidemiology
Recommended Textbook: Epidemiology 4th edition – Leon Gordis
Estimating Risk: Is There an
Association?
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12/13/2022
Outline
The concept of absolute risk
Compare relative risk and odds ratio
Calculate and interpret a relative risk and odds ratio.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Measures of disease comparison
 Measures
of disease frequency can be
compared in two ways:
They
can be subtracted from one another
OR
Divided
by one another
 The
subtracted measures of disease
frequency are terms absolute comparisons
 The
divided ones are known as relative
comparisons
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Measures of Comparison (Absolute)



Absolute comparisons: Risk/Rate difference can be
calculated either for the exposed individuals or for
the total population. In both cases, it takes the form
“A-B”. Absolute difference measure is calculated as:
RD = Rexposed - Runexposed R= Risk/Rate & D= Difference
RD describes the disease burden associated with
exposure among people who are exposed.
 It’s
simply the excess risk/rate of disease associated
with exposure
 It
describes the public health impact of the exposure
among the exposed.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Absolute risk

The incidence of a disease in a population is termed
the absolute risk.

Absolute risk can indicate the magnitude of the risk in
a group of people with a certain exposure,

But because it does not take into consideration the risk
of disease in non-exposed individuals, it does not
indicate whether the exposure is associated with an
increased risk of the disease.

Comparison is fundamental to epidemiology.

Nevertheless, absolute risk may have important
implications in both clinical and public health policy.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Ways of calculating excess risk
• This is the results of a hypothetical investigation of a foodborne disease
outbreak.
• The suspect foods were identified, and for each food, the attack rate (or
incidence rate) of the disease was calculated for those who ate the food
(exposed) and for those who did not eat the food (non-exposed)
RR
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
RD
Measures of comparison(Relative)

Relative comparisons: Risk/rate ratio gives
information on the strength of the relationship
between the exposure and the disease. It takes the
form “A/B”. Rate or Risk ratio is mathematically
expressed as:

Relative Risk = Rexposed/Runexposed where R= Risk or Rate

Relative Risk (RR) indicates how many times higher or
lower the disease risk is among the exposed as
compared to the baseline risk among the unexposed
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Relative risk

If we carry out a cohort study, we ask the question:

“What is the ratio of the risk of disease in exposed individuals to
the risk of disease in non-exposed individuals?”

This ratio is called the relative risk:

Relative risk is the ratio of the probability of an outcome in
an exposed group to the probability of an outcome in an
unexposed group.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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12/13/2022
Interpreting the Relative Risk
The relative risk is important as a measure of the strength of
the association, which is a major consideration in deriving
causal inferences
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2021-22 Academic Yr.
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Risk Calculation in a Cohort Study
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2021-22 Academic Yr.
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Example:
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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The odds ratio




In a case-control study, we do not know the incidence
in the exposed population or the incidence in the nonexposed population because we start with diseased
people (cases) and non-diseased people (controls).
Hence, in a case-control study we cannot calculate the
relative risk directly.
Another measure of association, the odds ratio, can be
used instead of the relative risk.
Even though we cannot calculate a relative risk from a
case-control study, under many conditions, we can
obtain a very good estimate of the relative risk from a
case-control study using the odds ratio
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Defining the Odds Ratio in Cohort and in
Case-Control Studies
• Odds of an event can be defined as the ratio of the number of ways the
event can occur to the number of ways the event cannot occur
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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The Odds Ratio in Cohort Studies
The odds ratio is also known as the cross-products ratio,
because it can be obtained by multiplying both diagonal cells in
a 2 × 2 table and then dividing
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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12/13/2022
Interpreting the Odds Ratio

We interpret the odds ratio just as we interpreted
the relative risk.

If the exposure is not related to the disease, the
odds ratio will equal 1.

If the exposure is positively related to the disease,
the odds ratio will be greater than 1.

If the exposure is negatively related to the disease,
the odds ratio will be less than 1.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
When is the odds ratio a good estimate of
the relative risk?

Odds ratio obtained in a case-control study is a good approximation
of the relative risk in the population when the following three
conditions are met:
1.
When the cases studied are representative, with regard to history
of exposure, of all people with the disease in the population from
which the cases were drawn.
2.
When the controls studied are representative, with regard to
history of exposure, of all people without the disease in the
population from which the cases were drawn.
3.
When the disease being studied does not occur frequently.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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12/13/2022
When is the odds ratio a good estimate of
the relative risk?

These two examples of cohort studies
demonstrate how the odds ratio provides a
good approximation of the relative risk

In Figure 11-6, the occurrence of disease is
infrequent and we see that the relative risk is
2. If we now calculate an odds (cross
products) ratio, we find it to be 2.02, which is
a very close approximation.

Now, let us examine Figure 11-7, in which the
occurrence of disease is frequent. Although
the relative risk is again 2.0, the odds ratio is
3.0, which is considerably different from the
relative risk.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Attributable risk

How much of the disease that occurs can be attributed to a
certain exposure?
 This
is answered by a measure of risk called the
Attributable Risk (AR)
 AR
is defined as the amount or proportion of disease
incidence (or disease risk) that can be attributed to a
specific exposure.

The attributable risk is in many ways more important in
clinical practice than in public health, because it addresses a
different question:
 How
much of the risk (incidence) of disease can we hope
to prevent if we are able to eliminate exposure to the
agent in question?
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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12/13/2022
Attributable risk
The attributable risk indicates the potential for prevention if
the exposure could be eliminated.
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2021-22 Academic Yr.
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Attributable risk
AR = Ie – Io or CIe – CIo
Dr. LMC -
Ie = incidence in exposed
Io = incidence in non exposed
CIe = cumulative incidence in exposed
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Academic Yr.
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= cumulative
in non exposed
Attributable Risk for the Total Population—
Population Attributable Risk (PAR)

What proportion of the disease incidence in a total population
(including both exposed and non-exposed people) can be
attributed to a specific exposure?

What would be the total impact of a prevention program on the
community?
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Population Attributable Risk (PAR)

The population attributable risk is a valuable concept for the
public health worker.

The question addressed is: What proportion of lung cancer in
the total population can be attributed to smoking?

This question could be reworded as follows: If smoking were
eliminated, what proportion of the incidence of lung cancer in
the total population (which consists of both smokers and
nonsmokers) would be prevented?

The answer is: the attributable risk in the total population
also called the population attributable risk—or the PAR
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2021-22 Academic Yr.
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© Dr. Ceesay 2018 @ UTG – Department of Public Health
12/13/2022
Example of an attributable risk calculation
• What does this mean? It means that 10.6 of the 28/1000 incident cases in smokers are
attributable to the fact that these people smoke.
• Stated another way, if we had an effective smoking cessation campaign, we could
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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hope to prevent 10.6 of the 28/1,000 incident cases of CHD that smokers experience.
Example of an attributable risk calculation
 Thus,
37.9% of the morbidity from CHD among
smokers may be attributable to smoking and
could presumably be prevented by eliminating
smoking.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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E.g. of Population Attributable Risk calculation

In this example, we know that the incidence among
the smokers is 28.0 per 1,000 and the incidence
among the nonsmokers is 17.4 per 1,000.

However, we do not know the incidence in the total
population.

Let us assume that, from some other source of
information, we know that the proportion of
smokers in the population is 44% (and therefore the
proportion of nonsmokers is 56%).

The incidence in the total population can then be
calculated as shown slide:
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
E.g. of Population Attributable Risk calculation
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
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E.g. of Population Attributable Risk calculation
Thus, 21.3% of the incidence of CHD in the total population
can be attributed to smoking, and if an effective prevention
program eliminated smoking, the best that we could hope to
achieve would be a reduction of 21.3% in the incidence of CHD
in the total population.
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022
Dr. LMC - UTG@Dept. of Nursing & Rep. Health
2021-22 Academic Yr.
1st Semester
12/13/2022