Case-control studies are observational studies, where two groups determine the level of exposure to a risk or a disease, by identifying a group of individuals with disease and for purpose of comparison, a group of people without the disease.

1. Medical laboratory specialist and public health student
dr.saleh14@gmail.com
Case-Control Study

2. C A s
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3. Case-control studies are observational studies, where two groups determine the level of
exposure to a risk or a disease, by identifying a group of individuals with disease and for
purpose of comparison, a group of people without the disease.
Subjects with disease (or outcome) are called cases and subjects without disease (or
outcome) are called controls.

4. • Cases
– All those with disease or outcome
– Defined population
• Controls
– Those without disease or outcome
– Sample of source population that gave rise to cases
• Comparison
– Measure exposure of interest
– Compare proportion exposed in cases to controls

5. Case-Control Study DeSIGN
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






exposed to a risk
Factor (a)
unexposed to a
risk Factor (b)
exposed to a risk
Factor (c)
unexposed to a
risk Factor (d)
Past Present FutureTime Study Begins Here
• The investigator collects retrospective information about exposure to the risk factor from both groups.

6. Exposed to
suspected risk
factor
Disease under investigation
TotalCase
(Disease Present)
Control
(Disease Absent)
Yes - Exposed a b a + b
No - Not Exposed c d c + d
Total a + c b + d a + b + c + d
𝐚
𝐜
𝐛
𝐝
𝐚
𝐜
÷
𝐛
𝐝
𝐚𝐝
𝐛c

7. How will find the strength of association between the disease under
investigation and the suspected risk factor?
Is the number (a) of observations in a given group with a given characteristic divided
by the number (b) of observations without the given characteristic.
The probability that an event will occur divided by the probability that the event will
not occur.
Is the ODDS that a patient is exposed to the risk factor divided by the ODDS that a
control is exposed
ODDS
+
Ratio
=
Odds
Ratio

8. OR < 1 OR = 1 OR > 1
Odds comparison
between cases and
controls
Odds Ratio for cases
are less than the Odds
Ratio for controls
Odds Ratio are equal
among cases and
controls
Odds Ratio for cases
are greater than the
Odds Ratio 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)

9. Exposed to
suspected risk
factor
Disease under investigation
TotalStroke
(Disease Present)
Control
(Disease Absent)
Yes - Exposed 73 18 a + b
No - Not Exposed 141 196 c + d
Total 214 214 a + b + c + d
𝟎. 𝟓𝟏𝟖 𝟎. 𝟎𝟗𝟐
𝟎. 𝟓𝟏𝟖 ÷ 𝟎. 𝟎𝟗𝟐 or =
𝐚𝐝
𝐛c
= 𝟓. 𝟔𝟒
• Example
The Odds is
That is, a person in this study who abuses drugs is
more than five times more likely to have a stroke.

10. - Cigarette smoking and lung cancer.
- Maternal smoking and congental malformations.
- Radiation and leukemia.
- Induces abortion and spontaneous abortion.
- Physical activity and coronary death.
- Artificial sweetner and bladder cancer.

11. Strengths and weaknesses of case-control studies
• Case-control studies cost less than other studies, e.g. cohort studies.
• Case-control studies are mostly retrospective; cases are identified at the beginning. There is no
need to follow cases over a period of time, unlike cohort studies.
• Case-control studies are more appropriate for rare diseases.
• The association between diseases and multiple exposures can be studied at the same time.
• Case-controls are subject to multiple biases (especially selection and recall biases).
• Case-control studies cannot estimate the incidence rate unless the rates are population based.
• Case-control studies are difficult for determining the time period between the exposure and disease.

12. Case-Control Study
- Jayaramachandran, Epidemiological exercises pdf.
- https://www2.slideshare.net/tarekksalem1966/odds-ratios-basic-concepts?from_action=save
- Hennekens CH, Buring JE. Epidemiology in Medicine, Lippincott Williams & Wilkins, 1987.
https://www.healthknowledge.org.uk/e-learning/epidemiology/practitioners/introduction-study-
design-ccs
- J Can Acad Child Adolesc Psychiatry. 2010 Aug; 19(3): 227–229
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2938757/
- Dr. Karim Abawi, Case-control studies From Research to Practice: Training in Sexual and
Reproductive Health Research 2017