This document provides an overview of case-control surveillance studies. It discusses that case-control studies are retrospective and aim to determine if an exposure is associated with an outcome. The document outlines how cases and controls are selected and defined, and how odds ratios are used to interpret the association between an exposure and disease. It also discusses potential sources of bias and confounding, and notes some key advantages and disadvantages of the case-control study design.

1. Case control Surveillance
Presented by: Manij Joshi, Md. Khurshid Ansari and Sandeep Raj Pandeya

2. Introduction
 A case-control study is designed to help determine if an exposure is associated
with an outcome.
 It is always retrospective because it starts with an outcome then traces back to
investigate exposures.
 Case-control studies assess whether exposure is disproportionately distributed
between the cases and controls, which may indicate that the exposure is a risk
factor for the health outcome under study.
 Case-control studies are frequently used for studying rare health outcomes or
diseases.

4. Suspected / risk factors
(High fat diet)
Cases (Colon cancer present) Control (Colon cancer
absent)
Present a b
Absent c d
a + c b + d
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. Controls
were a sample of residents without colon cancer.
Odds of exposure among cases = a/c
Odds of exposure among control = b/d
Now, Odds Ratio (OR)= (a/c)/(b/d)
Interpreting the Odds Ratio
OR = 1 Odds of disease is the same for exposed and unexposed
OR > 1 Exposure increases odds of disease
OR < 1 Exposure reduces odds of disease

5. Let us suppose,
 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. In another study of colon cancer and coffee
consumption, the OR was 0.60. Thus, the odds of colon cancer among coffee
drinkers is only 0.60 times the odds among individuals who do not consume coffee.
This OR tells us that coffee consumption seems to be protective against colon
cancer.

6. Basic Steps in conducting a Case Control Study
Selection of cases and controls
Matching
Measurement of exposure
Analysis and interpretation

7. 1. Selection of Cases and Controls
 Identify a suitable group of cases and a group of controls
 Comparability of cases and controls is essential

8. Selection of Cases
 Definition of disease
1) Diagnostic criteria
Once the diagnostic criteria are established, they should not be altered or
changed till the study is over.
2) Eligibility criteria
Eg: Age
 Sources of cases
1. Hospital
2. General Population

9. Selection of Control
Must be
 free from disease under study
 as similar to case as possible
 Sources of Controls
 Hospital Control
 Relatives
 Neighborhood control
 General population

10. Hospital Controls
Advantages
 Easily identified and readily available.
 Likely to have been subjected to the same selection factor.
 More likely to be willing to co‐operate than healthy individuals, minimizing bias
due to non‐response.
Disadvantage
They are ill and therefore differ from healthy individuals in a number of ways
May be associated with illness and hospitalization in general.

11. Relatives Control
 Advantages
 Healthy and co‐operative
 Offer a degree of control of important confounding factors related to ethnic
background, socio‐economic status or environment

12. Neighborhood Controls
 Controls may also be drawn from persons living in the same locality, working in
same factory, attending same school as cases.

13. General Population
 From defined geographical areas
Disadvantages:
 more costly and time consuming
 difficult to contact
 quality of information may differ
 individuals refuse to participate

14. 2. Matching
 A major concern in conducting a case‐control study is that cases and controls may
differ in characteristics or exposures other than the one that has been targeted for
study.
 e.g. if most of the cases are poor and most of the controls are affluent, we would not
know whether the factor determining development of disease is exposure to the
factor being studied or another characteristic associated with being poor.
 Matching is defined as the process of selecting the controls so that they are similar to
the cases in certain characteristics, such as age, race, sex, socioeconomic status, and
occupation.
 If not adequately matched, could distort or confound the results.

15.  Group matching (or frequency matching): Proportion of controls with a certain
characteristic is identical to the proportion of cases with the same characteristic.
e.g. if 25% of the cases are married, the controls will be selected so that 25% of that group is
also married.
 Individual matching (or matched pairs): A control is selected who is similar to the
case in terms of the specific variable or variables of concern. e.g., if the first case
enrolled in our study is a 25‐year‐old female, we will seek a 25‐year‐old female
control.

16. What are the problems with matching
1. Practical Problems with Matching:
 Match according to too many characteristics, it may prove difficult or impossible
to identify an appropriate control. e.g., match each case for race, sex, age,
marital status, number of children, zip code of residence, and occupation.
2. Conceptual Problems with Matching:
 Once we have matched controls to cases according to a given characteristic, we
cannot study that characteristic. For example, suppose we are interested in
studying marital status as a risk factor for breast cancer.

17. 3. Measurement of Exposure
 Definition and criteria for exposure
 Obtain by
interviews,
questionnaire,
past records etc.

18. 4. Analysis
To find out
 Exposure rates among cases and controls
 Estimation of disease risk associated with exposure (Odds Ratio)

19. What is Bias?
 “Any systemic error in the design, conduct or analysis of a study that results in a
mistaken estimate of an exposure’s effect on the risk of disease.”
Some sources of bias are:
Selection bias
Recall bias
Reporting bias
Interviewer’s bias

20. What is confounding?
 “A third variable or a mediator variable, can adversely affect the relation between
exposure and outcome.”

21. Advantages of Case Control Surveillance
 Easy to carry out.
 Inexpensive
 Short term
 Powerful even with small sample population.
 Suitable to investigate rare diseases.
 No risk to the subjects.
 Allows study of etiological factors.
 Risk factors can be identified.
 If properly performed (i.e. appropriate sampling), case-control studies provide
information that mirrors what could be learned from a cohort study, usually at
considerably less cost and time.

22. Disadvantages of Case Control
Surveillance
 Problems of bias and confounding.
 Selection of control group is difficult.
 Can't measure incidence.
 Not suited to evaluation of therapy or prophylaxis of disease.

23. Nested Case control studies
 It is an important type of case-control study where cases and controls are drawn
from the population in a fully enumerated cohort.
 Baseline characteristics of these patients are obtained by interview, physical
examination, and pertinent lab or imaging studies. The patients are then followed
to determine the outcome.
 Those patients who develop the condition of interest become cases in a case-
control study, and those who do not develop the condition become eligible for the
control group of a study.
 The cases and representative sample of controls are studied, and data from the
two groups are compared using analytic methods appropriate for case-control
studies

24. Design of Nested case control study

26. References
 https://www.healthknowledge.org.uk/e-
learning/epidemiology/practitioners/introduction-study-design-ccs
 Susan Lewallen,Paul Courtright (1998) 'Epidemiology in Practice: Case-Control
Studies', , 11(28), pp. 57-58.
 https://www.students4bestevidence.net/case-control-and-cohort-studies-
overview/
 https://www.britannica.com/science/case-control-study

27. Any Queries??