This document summarizes case control and ecological study designs. It describes case control studies as retrospective studies that compare cases (people with a disease) to controls (people without the disease) to determine associations between exposures and outcomes. Key aspects of case control studies discussed include selection of cases and controls, matching, measuring exposure, analyzing exposure rates and estimating risk through odds ratios. Ecological studies are described as comparing aggregate exposure and disease rates between populations without examining individuals. An example ecological study on water fluoridation and dental caries is provided.

1. CASE CONTROL
&
ECOLOGICAL STUDY
Presented By:
Jagan Kumar Ojha
M.Sc. in Community Health Nursing
SUM Nursing College

2. CASE CONTROL
A study that compares patients who have a disease or outcome of
interest (cases) with patients who do not have the disease or outcome
(controls), and looks back retrospectively to compare how frequently
the exposure to a risk factor is present in each group to determine the
relationship between the risk factor and the disease.

3. Case control studies are also known as "retrospective
studies" and "case-referent studies.
These studies are designed to estimate odds. The odds of an
event represent the ratio of the (probability that the event
will occur) / (probability that the event will not occur)
CASE CONTROL

4. • Case : A person in the population or study group
identified as having the particular disease, health
disorder or condition under investigation.
• Control: Person or persons in a comparison group that
differs, in disease experience (or other health related
outcome) in not having the outcome being studied.

5. Features of case controls study
• Both exposure and outcome has happened
before the start of the study.
• The study proceeds backwards from effect to
cause.
• It uses a comparison group to support or
refute an inference.

6. Design of a case-control study
Hallmark of Case Control Study:
from cases and controls and searches for exposure.

8. Steps in Case Control Study
• Selection of cases and controls
• Matching
• Measurement of exposure
• Analysis and interpretation

9. Selection of cases
• Definition of case
– Diagnostic criteria
– Eligibility criteria
• Sources of cases
Hospitals
General population
1. Selection of cases and controls

10. Selection of controls
• Controls must be free from disease under study
• Must be similar to the cases except for the disease
under study
• Selection of controls is the most difficult
• Sources of controls
– Hospitals
– General population
– Relatives
– Neighborhood

11. 2. Matching
• Matching is a process in which we select controls in
such a way that they are similar to cases with regard to
certain pertinent variables (eg. age) which are known to
influence the outcome of disease and which if not
adequately matched for comparability could
misrepresent or confound the results.

12. What is a confounding factor?
Esophageal cancer
Alcohol
Smoking

13. 3. Measurement of exposure
• Interviews
• Questionnaires
• Past records
– Hospital records
– Employment records

14. Analysis
• Find out
– Exposure rates among cases and controls to
suspected factor
– Estimation of disease risk associated with
exposure ( Odds Ratio)

15. Exposure rates
Cases
(with Ca Lung)
Controls ( without
Ca lung)
Total
Smokers
( <5/day)
33
(a)
55
(b)
88
(a+b)
Non-smokers 2
(c)
27
(d)
29
(c+d)
Total 35
(a+c)
82
(b+d)
117
(a+b+c+d)
A case control study between smoking and lungcancer

16. Exposure rates
A case control study between smoking and lungcancer
Cases
(with Ca Lung)
Controls ( without
Ca lung)
Total
Smokers
( <5/day)
33
(a)
55
(b)
88
(a+b)
Non-smokers 2
(c)
27
(d)
29
(c+d)
Total 35
(a+c)
82
(b+d)
117
(a+b+c+d)

17. Exposure rates
• Cases= a/(a+c)= 33/35= 94.2 %
• Controls= b/(b+d)= 55/82= 67 %
• So frequency of smoking was definitely higher
among lung cancer patients than those
without cancer

18. • Odds Ratio / Relative odds
– Odds: Odds of an event is defined as the ratio of the
number of ways an event can occur to the number of
ways an event cannot occur.
– Odds ratio: Ratio of the odds that the cases were
exposed to the odds that the controls were exposed.

19. • Odds ratio:
Odds that case was exposed
Odds ratio =
Odds that control was exposed
= (a/c)/ (b/d) = ad / bc
Outcomes of Case Control Study
Diseased/ Cases Not diseased/
Controls
Exposed a b
Not exposed c d

20. Estimation of risk
• Odds that cases were exposed= a/c
• Odds that controls were exposed=
b/d
• Odds ratio= (a/c)/(b/d)= ad/bc= 8.1

21. Interpretation
• Smoking was found be associated
8.1 times more in patients with lung cancer
than those without lung cancer.

22. ODD Ratio Interpretation
For example, investigators conducted a case-control study to determine
if there is an association between colon cancer and a high fat diet.
Cases were all confirmed colon cancer cases in North Carolina in 2010.
Controls were a sample of North Carolina residents without colon
cancer. The odds ratio was 4.0. This odds ratio tells us that individuals
who consumed a high fat diet have four times the odds of colon cancer
than do individuals who do not consume a high-fat diet.

23. Advantages
• Good for studying rare conditions or diseases
• Less time needed to conduct the study because the
condition or disease has already occurred
• Simultaneously look at multiple risk factors
• Can answer questions that could not be answered
through other study designs

24. Disadvantages
• Retrospective studies have more problems with data quality
because they rely on memory and people with a condition will be
more motivated to recall risk factors (also called recall bias).
• It can be difficult to find a suitable control group
• Incidence cannot be measured
• Not suitable for evaluation of Rx.

25. Ecological study

26. Ecological study

27. Ecological study
• Ecologic (correlational) Studies Compare rates of exposures and
diseases in different populations using aggregate data not individual
data.
Ecologic Studies Examples: –Very strong positive correlations were
found between meat and fat consumption and the rates of breast and
colon cancer in various countries.
Example, it was recognized that the use of water supplies with high
fluoride concentrations led to mottling of tooth enamel. Dentists
observed that people with mottled teeth had low rates of dental caries
and they hypothesized that fluoride might prevent caries. The US
Public Health Service tested this hypothesis by conducting an
ecological study.

28. Ecological study
Researchers surveyed the dental health of children in 13 cities where
the fluoride concentration in the water supply varied considerably. The
results, reported in 1942, indicated that dental caries decreased with
increasing fluoride content of water. A fluoride level of 1 ppm appeared
to be optimal; associated with a reduction in caries without an
unsightly degree of mottling.

29. SUMMARIZATION

30. THANK YOU