This document discusses case-control study design and calculating odds ratios. It provides examples of 3 case-control studies examining suspected risk factors for cervical cancer, lung cancer, and esophageal cancer. For each study, it constructs a 2x2 contingency table and calculates the odds ratio to assess the strength of association between the disease and suspected risk factor. Odds ratios greater than 1 indicate the exposure increases disease risk.

1. Epidemiological exercises
Dr Jayaramachandran S
Asst. Prof
Dept. of Community Medicine
MGMC & RI

2. Population
(a+b+c+d)
Diseased
(Cases : a+b)
Non Diseased
(Controls : c+d)
Exposed (a)
Not Exposed (c)
Exposed (b)
Not Exposed (d)
Case Control Study Design
6-Mar-19Epidemiological Exercises2

3. 2 × 2 Contingency table
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Exposed to
suspected RF
Disease under investigation
Total
Present Absent
Yes a b a + b
No c d c + d
Total a +c b + d a+b+c+d
How will find the strength of association
between the
disease under investigation and the suspected risk factor?

4. Odds Ratio (OR)
} Odds ratio is the strength of association between the risk
factor and outcome.
} Assumptions:
} The disease being investigated must be relatively rare
} Cases must be representative of those with the disease
} Controls must be representative of those without the disease
} OR =
} OR = ÷ =
6-Mar-19Epidemiological Exercises4
Odds of exposure among exposed (cases)
Odds of exposure among non – exposed (controls)
a
b
c
d
ad
bc

5. Interpretation of Odds ratio
OR = 1 OR > 1
Odds
comparison
between cases
and controls
OR for cases
are less than
the OR for
controls
OR are equal
among cases
and controls
OR for cases
are greater
than the OR
for controls
Exposure as a
risk factor for
the disease?
Exposure
reduces
disease risk
(Protective
factor)
Particular
exposure is
not a risk
factor
Exposure
increases
disease risk
(Risk factor)
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6. Problem 1
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A study was carried out to find if there is a causal association
between taking oral contraceptive pills (OCP) and developing
cervical cancer. 80 women with cervical cancer and 120
women without cervical cancer were enquired about history
of OCP use. 30 of the former group and 20 of the latter group
and gave a positive history of OCP usage.
1. What type of study design is this? Justify your answer. (1+1)
2. Construct a 2x2 table for the above data (3 marks)
3. Calculate the appropriate measure of strength of association.
(3 marks)
4. What is your interpretation of the results? (2 marks)

7. Problem 1
1. It is a case-control study, as cases and controls were selected
and details of history of exposure were enquired.
2. Table: Case-Control (2 ✕ 2 Table)
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Intake of OCP
Cervical cancer
Total
Present Absent
Yes 30 (a) 20 (b) 50
No 50 (c) 100 (d) 150
Total 80 120 200

8. Problem 1
3. Odd’s ratio = ad / bc
= 30 × 100 / 50 × 20
= 3
Yes, OCP usage is associated with increased development of
cervical cancer
} Odds of cervical cancer was 3 times higher among OCP users
compared with controls. We can interpret this in a simpler way
if the cervical cancer is rare and controls are representative of
the same population from where the cases are taken; we can
say that the risk of getting cervical cancer in OCP users is 3
times higher as compared with OCP nonusers.
6-Mar-19Epidemiological Exercises8

9. Problem 2
A study was conducted to find out if there was an association
between cigarette smoking and developing lung cancer. 55
persons with lung cancer and 130 matched persons without
lung cancer were enquired about history of cigarette smoking
(> 5 cigarettes per day). 4 persons in former group and 45
persons in latter group gave a negative history.
1. What type of study design is this? Justify your answer. (1+1)
2. Construct a 2x2 table for the above data (3 marks)
3. Calculate the appropriate measure of strength of association.
(3 marks)
4. What is your interpretation of the results? (2 marks)
6-Mar-19Epidemiological Exercises9

10. Problem 2
1. It is a case-control study, as cases and controls were selected
and details of history of exposure were enquired.
2. Table: Case-Control (2 ✕ 2 Table)
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Smoker
Lung cancer
Total
Present Absent
Yes 51 (a) 85 (b) 136
No 4 (c) 45 (d) 49
Total 55 130 185

11. Problem 2
3. Odd’s ratio = ad / bc
= 51 × 45 / 4 × 85
= 6.75
Yes, Smoking is associated with increased development of
lung cancer
} Odds of smoking was 6.75 times among lung cancer cases
compared with controls. We can interpret this in a simpler way
if the lung cancer is rare and controls are representative of the
same population from where the cases are taken; we can say
that the risk of getting lung cancer in smokers is 6.75 times as
compared with non-smokers.
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12. Problem 3
A study was conducted to find the association between
alcohol and development of oesophageal cancer. In total, 300
cases of oesophageal cancer and 280 healthy persons were
enquired about the history of alcohol intake. 130 persons in
former group and 40 persons in latter group gave a positive
history of alcoholintake.
1. What type of study design is this? Justify your answer. (1+1)
2. Construct a 2x2 table for the above data (3 marks)
3. Calculate the appropriate measure of strength of association.
(3 marks)
4. What is your interpretation of the results? (2 marks)
6-Mar-1912 Epidemiological Exercises

13. Problem 3
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1. It is a case-control study, as cases and controls were selected
and details of history of exposure were enquired.
2. Table: Case-Control (2 ✕ 2 Table)
Blindness
Diabetes
Total
Present Absent
Present 130 (a) 40 (b) 170
Absent 170 (c) 240 (d) 410
Total 300 280 580

14. Problem 3
3. Odd’s ratio = ad / bc
= 130 × 240 / 40 × 170
= 4.6
4. Odds of alcohol intake was 4.6 times among oesophageal
cancer cases compared with controls. We can interpret this
in a simpler way if the oesophageal cancer is rare and
controls are representative of the same population from
where the cases are taken; we can say that the risk of getting
oesophageal cancer in alcohol takers is 4.6 times as
compared with alcoholnon-takers.
6-Mar-19Epidemiological Exercises14