1. Present: Disease Past: Exposure
CHP400:
Community Health Program- lI
STUDY DESIGNS
Observational / Analytical Studies
Time is Key
Research
Methodology
nbmmb
Present: Disease &
Exposure
Present: Exposure Future: Disease
Cross - section
Cohort
Case
Control
Studies
Case - control 
Present: Disease Past: Exposure

2. Content Definition and importance of Case - Control Studies Design
Selecting Cases
Selecting Controls
Assessing Exposure: Odds Ratio (OR)
Advantages
Disadvantages

3. Case Control Studies Research Methodology STUDY DESIGNS Experimental
Observational
Experimental
Animal
Experiment
Descriptive
Analytical
Case
Control
Studies
Case report
Case
control
Case series
Human
Intervention
Clinical trial
Cross section
Cohort
Ecological

4. Observational / Analytical Studies Case - Control Studies Definition:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Definition:
It is a type of observational analytic epidemiologic investigation in which subjects are selected on the basis of whether they do or do not have the particular disease under study.

5. Case - Control Studies Importance:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Importance:
The most frequently undertaken analytical epidemiological studies
The only practical approach for identifying risk factors for rare diseases
They are best suited to the study of diseases for which medical care is sought, such as cancers or hip fracture

6. Case - Control Studies Design : At baseline:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Design :
At baseline:
Selection of cases (disease) and controls (no disease) based on disease status
Exposure status is unknown
Retrospective design – lacks temporality!

7. Case - Control Studies Design : Exposed Diseased Not Exposed
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Design :
Exposed
Diseased
(Cases)
Not Exposed
Target Population
Exposed
Not Diseased
(Controls)
Not Exposed

8. Case - Control Studies Design :  ? YES ? NO
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Design : 
EXPOSURE
OUTCOME/DISEASE
TIME ?
YES ? NO

9. Case - Control Studies Selecting Cases:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Selecting Cases:
Select cases after the diagnostic criteria and definition of the disease is clearly established
Selected cases should be representative of all cases

10. Case - Control Studies Selecting Cases:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Selecting Cases:
The study need not include all cases in the population
Cases may be selected from hospitals, clinics, disease registries, screenings, etc.

11. Case - Control Studies Selecting Cases:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Selecting Cases:
Incident cases are preferable to prevalent cases for reducing
(a) recall bias and
(b) over-representation of cases of long duration
The most desirable way to obtain cases is to include all incident cases in a defined population over a specified period of time

12. Case - Control Studies Selecting Controls:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Selecting Controls:
Controls should come from the same population at risk for the disease as the cases
Controls should be representative of the target population

13. Case - Control Studies Selecting Controls:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Selecting Controls:
Multiple controls can be used to add statistical power when cases are difficult to obtain
Using more than one control group lends credibility to the results
More than 3 controls for a case is usually not cost-efficient

14. Sources of cases and controls
Selecting Controls:
Sources of cases and controls
CASES
CONTROLS
All cases diagnosed in the community
Sample of general population
Non-cases in a sample of the population
All cases diagnosed in a sample of the population
All cases diagnosed in all hospitals
Sample of patients in all hospitals who do not have the disease
All cases diagnosed in a single hospital
Sample of patients in the same hospital who do not have the disease
Any of the above methods
Spouses, siblings or associates of cases

15. Case - Control Studies Analysis: Odds Ratio (OR)
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Analysis:
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
Odds: the possibility that something will happen : the chance that one thing will happen instead of a different thing An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure. Odds ratios are most commonly used in case-control studies, however they can also be used in cross-sectional and cohort study designs as well (with some modifications and/or assumptions

16. Case - Control Studies Analysis: Odds Ratio (OR) Disease Status = = =
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Analysis:
Odds Ratio (OR)
Disease Status
CHD cases
(Cases)
No CHD
(Controls)
Smoker
112
176
Exposure Status a b
Non-smoker 88 c d
224
Total
200
400 a b ad
112 x 224
Odds Ratio = = = =
1.62 c d bc
176 x 88

17. Interpreting the Odds Ratio (e.g)
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 is not
a risk factor nor
a protective factor
Exposure as a risk factor for the disease?
Exposure increases disease risk (Risk factor)
Exposure reduces disease risk
(Protective factor)

18. Interpreting the Odds Ratio
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Analysis:
Odds Ratio (OR)
Interpreting the Odds Ratio
The odds of exposure for cases are 1.62 times the odds of exposure for controls.
Those with CHD are 1.62 times more likely to be smokers than those without CHD

19. Possible Sources of Bias and Error
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Possible Sources of Bias and Error
Information on the potential risk factor (exposure) may not be available:
- either from records
- or the study subjects’ memories

20. Case - Control Studies cont.
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies cont.
Cases may search for a cause for their disease and thereby be more likely to report an exposure than controls (recall bias)
The investigator may be unable to determine with certainty whether the suspected agent caused the disease or whether the occurrence of the disease caused the person to be exposed to the agent

21. Case - Control Studies cont.
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies cont.
Identifying and assembling a case group representative of all cases may be unduly difficult
Identifying and assembling an appropriate control group may be unduly difficult

22. Several exposures can be studied.
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Advantages:
Quick and easy to complete, cost effective
Most efficient design for rare diseases
Usually requires a smaller study population than a cohort study
Several exposures can be studied.

23. Case - Control Studies Disadvantages:
Research Methodology
STUDY DESIGNS
Observational / Analytical Studies
Case - Control Studies
Disadvantages:
Uncertainty of exposure-disease time relationship
Inability to provide a direct estimate of risk
Not suitable for studying rare exposures
Subject to biases (recall & selection bias)

24. Summary Case-control study: Observational, analytic study.
Most frequently undertaken analytical studies
Quick and easy to complete, cost effective
Most efficient design for rare diseases
Subjects are selected on the basis of presence or absence disease under study
Odds Ratio (OR)