2. Introduction
Suppose you are a clinician and you have
seen a few patients with a certain disease.
You observe that many of them have been
exposed to a particular agent—biological or
chemical.
You hypothesize that their exposure is
related to their risk of developing this
disease.
How would you go about confirming or
refuting your hypothesis?
Study designs!!!
3. CASE CONTROL
• Is an observational study in which the two
existing groups differing in outcome(case
and control) are identified and compared on
the basis of some supposed causal attribute.
• Study of persons with the disease (or another
outcome variable) of interest and a suitable
control group of persons without the disease
(comparison group, reference group)4.
5. Design
• Start with people who have disease(Cases)
• Match them with control that do not have the
disease
• Measure past exposure
• Measure: interview or by review of medical or
employee records or of results of chemical or
biologic assays of blood, urine or tissues.
8. Hypothetical Example-Case control
• Whether smoking is related to CHD.
• We start with 200 people with CHD (cases)
and compare them to 400 people without
CHD (controls)
• Of the 200 CHD cases, 112 were smokers
and 88 were nonsmokers.
• Of the 400 controls, 176 were smokers
and 224 were nonsmokers
9. Hypothetical Example-Case control…
• If there is a relationship between a lifetime
history of smoking and CHD.
• Greater proportion of the CHD cases than
of the controls would have been smokers
(exposed).
• Our example: 56% of CHD cases were
smokers compared to 44% of the controls.
10. Sources of control group
• Non hospitalized
• OPD
• Inpatients admitted for diseases other than
that for which the cases were admitted.
11. When to use it
• First step when searching for a cause of an adverse health
outcome.
• Case-control studies are also valuable when the disease
being investigated is rare.
• Out break
Eg: In a study addressing the association of Guillain-
Barré syndrome with Zika virus infection in French Polynesia in
2013–2014;Cao-Lormeau and colleagues noted that during the
Zika outbreak, there was an increase in reports of Guillain-Barré
syndrome suggestive of a possible relationship
12. Advantages
• Generally less expensive-than cohort studies.
• Can be carried out more quickly.
• Case-control studies are also valuable when
the disease being investigated is rare.
13. Disadvantages
• Potential for recall bias
• Potential for selection bias
• Can not establish risk
• Can not establish prevalence
14. Possible biases
• Selection bias
-Sources of cases eg. Tertiary hospital
-Using incident or prevalent case
Preferable incident cases; any risk factors we may
identify in a study using prevalent cases may be related
more to survival with the disease than to the
development of the disease (incidence)
• Information bias
-Recall problems,two types: limitations in recall
and recall bias
16. Recall Bias
• Individuals with a particular disease tend to
remember every detail of exposure they might
have had that has led them to have the disease
• Eg.Study on covid 19 pt Vs non Covid 19 with
exposure being in same room with a person
coughing
• Those with cvd will remember more while those
without might miss or not at all recall
Ernst Wynder,a well-known epidemiologist, also
called it “rumination bias.”
17. Summary
CASE CONTROL
• What is it
• How to design
• Sources of control group
• When to use it
• Advantages/Disadvantages
• Biases
18. A case-control study is characterized by all of the
following except:
a. It is relatively inexpensive compared with
most other epidemiologic study designs
b. Patients with the disease (cases) are
compared with persons without the disease
(controls)
c. Incidence rates may be computed directly
d. Assessment of past exposure may be biased
e. Definition of cases may be difficult
19. References
1. Deakin Univeristy Lirary. [Online]
https://deakin.libguides.com/quantitative-study-
designs/casecontrol.
2. Frontiers for young Minds. [Online]
https://kids.frontiersin.org/articles/10.3389/frym.2020.00015
3. David, Moyses Szklo. Case control Studies. GORDIS
EPIDEMIOLOGY, SIXTH EDITION. Philadelphia : Elyse O’Grady.
4. Porta, Miquel. [book auth.] International Epidemiological
Association. A dictionary of epidemiology 6th Edition. s.l. :
International Epidemiological Association, Inc.
–sp.plan/protocol for conducting a study which allows the investigator to translate conceptual hypothesis into an operational one
Of the 42 patients with Guillian-Barré syndrome, 98% (41/42) had antibodies against the Zika virus, compared with 56% of controls
In epidemioloical study we have Selection bias,Information bias and confounding
Details that we might have otherwise erase from our memory,once we are ill we often remember as when we are not.
Can be prevented by using objective ways to assess exposure such as biomarkers or assays.
Differential misclassification Vs non differential
