This document outlines the key aspects of a case-control study design. It discusses the steps in conducting a case-control study, including selecting cases and controls, measuring exposure, and analyzing the data. Important concepts covered include matching, odds ratios, biases that can occur, and the advantages and disadvantages of the case-control study design. The overall goal of the study is to evaluate associations between exposures and disease outcomes.

1. Dr. Animesh Gupta
MD,FDM,FAGE
Associate Professor, Dept. of Community Medicine, NMCH, Sasaram (Bihar)
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2. At the end of the class, the students should know
▪ Different types of epidemiological methods
▪ Salient features of case control study
▪ Steps for conducting case control study
▪ Matching
▪ Odds ratio
▪ Bias in case control study
▪ Advantages & disadvantages in case control study
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3. OBSERVATIONAL STUDIES
DESCRIPTIVE STUDY ANALYTICAL STUDIES
✓ TIME
✓ PLACE
✓ PERSON
➢ ECOLOGICAL STUDY
➢ CROSS SECTIONAL STUDY
➢ CASE-CONTROL STUDY
➢ COHORT STUDY
EXPEREMENTAL STUDIES
➢ RANDOMIZED CONTROLLED TRIAL (RCT)
➢FIELD TRIAL
➢COMMUNITY TRIAL 4/25/2020Case Control Study Dr Animesh Gupta 3

4. ▪Makes careful measurement of patterns of exposure and
disease in population
▪Comparison group
▪The object is not to formulate but to test the hypothesis.
▪To evaluate an association between exposure and disease.
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5. ▪The observational epidemiologic study of persons with the
disease (or other outcome variable) of interest and a
suitable control (comparison/ reference) group of persons
without the disease. (- John M Last)
▪It is an observational study in which subjects are sampled
based upon presence or absence of disease and then their
prior exposure status is determined.
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6. ▪ Also knowns as “retrospective study”
▪ From effect to cause
▪ The case control method has three distinct features:
▪ Both exposure and out come have occurred before the start of
the study.
▪ The study proceeds backwards from effect to cause.
▪ It uses a control or comparison group to support or refute
an inference.
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8. 1. Selection of cases and controls
2. Matching
3. Measurement of exposure
4. Analysis & interpretation
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9. ▪Study begins with cases, i.e. the patients in whom the disease
has already occurred.
▪Cases were enquired for all the details of their exposure to the
suspected cause.
▪Definition of case:
▪ Diagnostic criteria : Clear cut diagnostic criteria of the disease
and its stage.
▪ Eligibility criteria : It is always advisable to take the incident
cases,since the prevalent cases might have changed their
exposure status due to medical advice etc.
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10. ▪Sources of Cases
▪ Hospitals
▪ Single hospital or a network of hospitals, admitted during a specified
period of time.
▪ The entire case series or a random sample of it is selected for study.
▪ General population
▪ Defined geographic area during a specified period of time
▪ Can be taken data through a survey, a disease registry or hospital
network.
▪ The cases should be fairly representative of all cases in the
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11. ▪ Controls must be free from disease
▪ Must be similar to the cases as far as possible except for the
absence of disease.
▪ Sources:
▪ Hospital control
✓Different illness other than the study disease
▪ Relatives
✓Except genetic condition study
▪ Neighborhood
✓Same locality/ working in same area/factory
▪ General population
✓Defined geographic areas
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12. ▪Select controls from various diagnostic groups, so no
particular risk factors will be over represented
▪Select controls from patients with acute conditions, so
earlier exposures could not have been influenced by the
condition
▪Do not select patients who have multiple concurrent
conditions.
▪Do not select patients with diagnoses known to be related
to the risk factor of interest
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13. ▪ Matching is the process of selecting controls who
are similar to the cases in certain key
characteristics (eg.age,sex,race) which are
known to influence the outcome of the disease.
▪ Individual matching (matched pair)
▪ for each case, a control is selected who is similar to the
case for a given variable
▪Group matching (frequency matching)
▪ proportion of controls with a given characteristic
(variable) is identical to proportion of cases with the same
characteristic
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14. ▪Defined as one which is associated both with exposure and
disease and is distributed unequally in study and control
groups.
▪Alcohol & Oesophageal cancer study – smoking is a
confounding factor.
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15. To find out
▪Exposure rate among cases and controls to suspected
factor.
▪Estimation of disease risk associated with exposure (Odds
ratio).
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16. Cases Controls
Risk factor + a b
Risk factor -- c d
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17. ▪Among cases = a/a+c
▪Among controls = b/b+d
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18. ▪Odds = is the ratio of the probability that an event will
happen to the probability that it will not happen
▪Odds ratio / cross product ratio
= ad/bc
▪It interprets that odds of cases being exposed are so many
times higher compared to the odds of controls being
exposed.
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19. ▪Based on 3 assumption that
▪Disease being investigated must be relatively rare.
In fact majority of the chronic disease have a low incidence in
the general population.
▪The cases must be representative of those with the disease.
▪The controls must be representative of those without the
disease.
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20. It is systematic error in the determination of the association
between the exposure and the disease.
▪Bias due to confounding factor
▪Memory or recall bias
▪Selection bias
▪Interviewer’s / Observer bias
▪Berkesonian bias ( Dr Joseph Berkeson)
▪ Different rates of admission to the hospital with different
disease.
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21. Strength
▪ Good for examining rare
outcomes or outcomes with
long latency
▪ Relatively quick to conduct,
inexpensive
▪ Requires comparatively few
subjects
▪ Multiple exposures or risk
factors can be examined.
Weakness
▪ Bias
▪ Selection of an appropriate
control group may be
difficult
▪ Incidence cant be calculated
▪ Do not distinguish between
causes and associated
factors
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22. ▪It is usually conducted before a Cohort or Experimental
Study to identify the possible etiology of the disease.
▪ It costs relatively less and can be conducted in a shorter time
▪For a given disease, a case control study can investigate
multiple exposures (when the real exposure is not known)
▪Case control study is preferred when the disease is rare
because investigator can intentionally search for the cases.
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23. Thank You
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