1. Epidemiology MPH 531 Analytic Epidemiology Case control studies
Mpundu MKC
MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

2. Case control studies Definition
An analytic epidemiologic study design in which the study population consist of groups, one with the disease/condition and a group without the disease (controls) and compare the patterns of the exposure (risk factor)

3. Case-Control Studies Definition:
An observational analytic epidemiologic study which examines the association between exposure and a health outcome by comparing cases or individuals who develop outcome and controls who are a sample of the source population from which the cases were identified

4. Case-Control Studies
Two groups are selected, one of people with the disease (cases), and the other of people with the same general characteristics but without the disease (controls)
Compare the past exposures of both groups to determine if the exposure could account for the health condition in case

5. Case-Control studies Direction
Subjects selected on the basis of Disease
Start with persons having Disease
Compare frequency of Exposure in cases (Disease) with frequency of Exposure in controls (non-Disease)
Look backward for history of Exposure
Direction always backward
Exposure Disease

6. Case-Control Studies Timing
Review timing: Disease happened when study begin
Basic design: Retrospective
Disease as occurred before onset of study

7. Characteristics of Case-Controls
Observational study
Explanatory (Analytical)
Retrospective
Disease to Exposure
Both Exposure and Disease have already occurred
Uses comparison group

8. Case-Control Studies Disease ( 𝑫 + ) Not Exposed Source
Population Exposed
Controls ( 𝐷 − )

9. Possible Sources of Cases
Hospitals, Acute and emergency (Casualty)
Clinics, physician offices
Laboratories
Disease registries
Surveillance systems
Vital records (Central Statistical office, Council)
General population

10. Case-Control Studies Selection of cases
Case definition
Inclusion and exclusion criteria

11. e.g. of CASES Presence of disease, injury, disability or death
Delayed immunisation
Low birth weight
Drug Resistance (MDR-TB)

12. Case-Control Studies Control Selection
Controls should
Represent source population from which the cases were derived
Represent persons who, if a case, would have been in the study
Be selected independently of Exposure

13. Odds Ratio Calculation
Outcome
Exposure
Cases
Controls
Exposed A B C D
Not Exposed
estimates the relative risk
Odds of exposure for cases
A / C
=Odds Ratio
Odds of exposure for controls
B / D

14. Odds Ratio (OR)
A ratio that measures the odds of exposure for cases compared to controls
Odds of exposure
= number exposed number unexposed
OR Numerator: Odds of exposure for cases
OR Denominator: Odds of exposure for controls

15. Comparing Odds Ratios and Relative Risks
Outcome
Exposure
Cases
Controls
Exposed 70
300
370 30
700
730
Not Exposed
1100
100
1000

16. e.g. calculations
Relative Risk = 4.41
Odds Ratio = AD/BC = 5.44

17. Calculating the Odds Ratio
Disease Status
CHD cases
(Cases)
No CHD
(Control)
Exposure Status
Smokers
132
176 88
324
Non-smokers
Total
220
500

18. e.g of calculations Odds(smokers) =case/control=132/176
Odds(nosmokers)
=case/control=88/234

19. Odds Ratio = AD BC
= x 324
176 x 88
OR= 2.76

20. Case-control Studies Measures of Association
Odds Ratio also known as Ratio of Two Odds
Estimator of relative risk
Odds of Exposed among cases compared with Exposure among controls
Also known as exposure odds ratio
- OR = 1 – no association
- OR > 1 – Exposure is a risk factor
- OR < 1 – Exposure is protective

21. OR<1
OR=1
OR>1
Odds of exposure are equal among cases and controls
Odds of exposure for cases are greater than the odds of exposure for controls
Odds comparison between cases and controls
Odds of exposure for cases are less than the odds of exposure for controls
Exposure as a risk factor for the disease?
Particular exposure is not a risk factor
Exposure increases disease risk (Risk factor)
Exposure reduces disease risk
(Protective factor)

22. Case-control Studies Common Biases
Selection bias
Information bias
- recall bias
Confounding

23. Case Control Studies Advantages
Quicker, cheaper and require less time and effort than cohort studies
Case-control studies can study rare diseases
Case-control studies can study multiple risk factors/exposures
They are useful for studying outcomes (diseases) that take a long time to develop, e.g. cancer

24. Case CONTROL STUDIES Disadvantages
Case-control studies are prone to selection and recall bias (i.e. better recollection of exposure amongst cases than among controls
They are inefficient for examining rare exposures
It may be difficult to establish temporality (when the person was actually exposed to the disease/risk factor)

25. Case CONTROL STUDIES Disadvantages
It can be difficult to choose an appropriate control (comparison) group
Case-control studies cannot calculate incidence rates, relative risks or attributable risks. Instead odds ratio are the measure of association used (when outcome is uncommon, e.g. most cancers, it can be a good proxy for the true relative risk
Incomplete data

26. Summary Case-control Studies
One of the major types of analytic epidemiological study designs
Start with disease and look backward for exposure
Odds ratio is standard measure of association