1. CASE CONTROL STUDY
PRESENTER: DR ABIOYE O.D
FACILITATORS: DR ADIGUN
DR ADENIRAN
DEPARTMENT OF COMMUNITY MEDICINE
UNIOSUN TEACHING HOSPITAL OSOGBO
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2. OUTLINES
• Objectives
• Introduction
• Analytical study designs
• Design of a case control study
• Advantages of case control study
• Limitations to case control study
• Bias in case control study
• Outcomes of case control study
• Bibliography
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3. OBJECTIVES
• To develop an understanding of what case-control studies are
• Understand the value of such studies
• Know the basic methodology for case-control studies
• Know the pros and cons of case-control studies
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4. INTRODUCTION
• A study design is a specific plan or protocol for conducting the study,
which allows the investigator to translate the conceptual hypothesis
into an operational one
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5. QUANTITATIVE VERSUS QUALITATIVE STUDY
• Quantitative research is used to quantify the problem by way of
generating numerical data which can be transformed into useable
statistics. It can generalize results from sample to population.
• Qualitative research is exploratory research. It is used to gain an
understanding of underlying reasons, opinions etc. and provides
insight into the problem
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6. QUANTITATIVE STUDY
• OBSERVATIONAL STUDY
DESCRIPTIVE STUDY
Case series
Case report
ANALYTICAL STUDY
Group data- Ecological study
Individual data -Case control (Retrospective study)
-Cohort(Prospective study)
-Cross-sectional study
• EXPERIMENTAL STUDY
-Randomized control trial
-Non-randomized control trial
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7. EPIDEMIOLOGICAL STUDY CYCLE
• The sequence of events starting with description of disease or health
related event in relation to time, place and person
• It includes searching for and finding differences in occurrence in
different populations
• Then formulating hypotheses regarding possible causative factors and
testing them
• Followed by analysing the results
• The results may lead to further descriptive studies or new
hypotheses.
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9. STUDY DESIGNS
• Observational Studies do not involve any intervention or experiment
• Differences in study group is only observed and analysed
• Not experimentally created
• Experimental Studies entails manipulation of the study factor and
randomization of subjects to treatment groups
• Differences in study group is experimentally created and outcomes
observed
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12. CASE CONTROL STUDY
• A study that compares two groups of people: those with the disease
or condition under study (cases) and a very similar group of people
who do not have the disease or condition (controls)
• A case control study has three distinct features
• Both exposure and outcome 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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17. CASE CONTROL VERSUS COHORT STUDY
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Case Control Studies Cohort Studies
18. STEPS IN CONDUCTING A CASE CONTROL
STUDY
• Selection of cases and controls
• Matching
• Measurement of exposure
• Analysis and interpretation
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19. SELECTION OF CASES
• Ideally, cases are a random sample of all cases of interest in the source
population (e.g. from vital data, registry data)
• More commonly they are a selection of available cases from a medical
care facility. (e.g. from hospitals, clinics)
• Information can be collected from cases themselves, or from a respondent
by proxy (relative/ friend), from records or a combination of the above
• Selection of cases involves the use of diagnostic and eligibility criteria
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20. SELECTION OF CASES
• Selection is usually from incident rather than prevalent cases
• Incident cases are those derived from ongoing ascertainment of cases
over time
• Prevalent cases are derived from a cross-sectional survey
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21. SELECTION OF CONTROLS
• The control group should be representative of the general population in
terms of probability of exposure to the risk factor
• They should also have had the same opportunity to be exposed as the
cases have
• Not that both cases and controls are equally exposed; but only that they
have had the same opportunity for exposure
• Sources of controls includes hospitals, relatives, neighbourhood or general
population similar to that of the cases
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22. SELECTION OF CONTROLS
• The study base is composed of a population at risk of exposure over a period of time
• Cases emerge within a study base and controls should emerge from the same study
base, except that they are not cases
• For example, if cases are selected exclusively from hospitalized patients, controls must
also be selected from hospitalized patients
• The control should be at risk of the disease
• The control should resemble the case in all respects except for the presence of the
disease
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23. MATCHING
• Matching is defined as the process of selecting controls so that they
are similar to cases in certain characteristics such as age, sex, race,
socioeconomic status and occupation
• Matching is done in order to ensure comparability between cases and
controls
• If matching is not done, these variables could distort or confound the
results
• A confounding factor is one which is associated both with exposure
and disease and is distributed unequally in study and control groups
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24. MATCHING
• Matching can be by matching variables (e.g. age), and matching criteria
(e.g. within the same 5 year age group) must be set up in advance
• Controls can be individually matched (most common) or Frequency
matched
• Individual matching: search for one (or more) controls who have the
required matching criteria
• It is a form of Paired matching where there is one (two) control(s)
individually matched to each cases
• Group matching (Frequency matching): select a population of controls
such that the overall characteristics of the case, e.g. if 15% cases are under
age 20, 15% of the controls are also
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25. MEASUREMENT OF EXPOSURE
• The criteria for determining exposure (i.e the variable of aetiological
importance) are equally important
• Information about exposure should be obtained in precise manner as
it was done for cases and controls
• This may be done by interviews, questionnaires or by studying past
records such as hospital or employment records etc
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26. ANALYSIS OF CASE CONTROL STUDY
• On analysis of case control study we may find out;
• Exposure rates: the frequency of exposure to suspected risk factor in
cases and in controls
• Odds ratio: It is a measure of the strength of association between the
risk factor and the disease being studied
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27. EXPOSURE RATES
Exposure rates
Cases = a/ (a + c) = 33/ 35 = 94.2%
Controls = b/ (b + d) = 55/82 = 67.0%
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A case control study provides a direct estimation of the exposure rates (frequency
of exposure) to the suspected factor in disease and non-disease groups
30. INTERPRETING ODDS RATIO
• OR = 1
-Odds of exposure among cases and controls are same
-Exposure is not associated with disease
• OR > 1
-Odds of exposure among cases are higher than controls
-Exposure is positively associated with disease
• OR < 1
-Odds of exposure among cases are lower than controls
-Exposure is negatively associated with disease
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31. ADVANTAGES OF CASE CONTROL STUDY
• Only realistic study design for uncovering etiology in rare diseases
• Important in understanding new diseases
• Commonly used in outbreaks investigation
• Useful if disease incubation period is long
• Relatively inexpensive; cheap, easy and quick
• No risk to subjects
• Require comparatively few subjects
• Multiple exposures can be examined
• No problem of attrition because it does not require follow-up
• Suitable when randomization is unethical
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32. LIMITATIONS TO CASE CONTROL STUDY
• Susceptible to bias if not carefully designed
• Susceptible to exposure misclassification
• Especially susceptible to recall bias
• Restricted to single outcome
• Cannot calculate Incidence rates
• Cannot distinguish between causes and risk factors
• Cannot assess effects of matching variables
• Selection of cases and control can be cumbersome
• If the incidence of exposure is high, it is difficult to show the difference
between cases and controls
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33. BIAS IN CASE CONTROL STUDY
• BIAS – is a systematic error in a the design, conduct or analysis of a
study which leads us to an erroneous conclusion
• Types of bias in case control studies:
• Selection bias
• Information bias
• Confounding bias
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34. BIAS IN CASE CONTROL STUDY
• SELECTION BIAS- Bias in selection of cases
Sources includes;
• Selective loss to follow-up
• Incomplete ascertainment of cases (Detection or Diagnostic bias)
• Inappropriate control group
• Differential motivation to participate
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35. BIAS IN CASE CONTROL STUDY
• INFORMATION BIAS
Occurs due to -
• Imperfect definitions of study variables
OR
• Flawed data collection procedures.
• Leads to – Misclassification of disease and exposure.
• Types of Information bias –
• Recall bias
• Interviewer bias
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36. BIAS IN CASE CONTROL STUDY
• Recall bias: Cases who are aware of their disease status may be more
likely to recall exposures than controls e.g. congenital malformation
with prenatal infections
• Interviewer bias: When an interviewer is not blinded i.e knows the
case status of subjects, there is potential for interviewer bias
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37. SOME IMPORTANT FINDINGS OF CASE
CONTROL STUDY
1950’s
• Cigarette smoking and lung cancer
1970’s
• Diethyl stilbestrol and vaginal adenocarcinoma
• Post-menopausal estrogens and endometrial cancer
1980 ’s
• Aspirin and Reyes sydrome
• Tampon use and toxic shocks syndrome
• L-tryptopham and eosinophilia-myalgia syndrome
• AIDS and sexual practices
1990’s
• Vaccine effectiveness
• Diet and cancer
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38. BIBLIOGRAPHY
• Park, k.Park’sTextbook of Preventive and Social Medicine 25thed;2021.
• Mausner & Bahn Epidemiology: An Introductory Text 2nd ed; Mausner
JS, Kramer S. 2015.
• A Dictionary of Epidemiology 3rd ed; Last JM.2020.
• Epidemiology 3rd ed; Gordis L. 2019.
• Origins and early development of the case-control study by Nigel
Paneth, Ezra Susser, Mervyn Susser. Available from
www.epidemiology.ch/history/papers
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