5. Case control

This is the lecture for Case control study (HFS3063)

1. KNOWLEDGE FOR THE BENEFIT OF HUMANITYKNOWLEDGE FOR THE BENEFIT OF HUMANITY
PUBLIC HEALTH AND EPIDEMIOLOGY (HFS3063)
Epidemiological Study Designs:
CASE CONTROL
Dr. Dr. MohdMohd RazifRazif ShahrilShahril
School of Nutrition & Dietetics School of Nutrition & Dietetics
Faculty of Health SciencesFaculty of Health Sciences
UniversitiUniversiti Sultan Sultan ZainalZainal AbidinAbidin
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2. Topic Learning Outcomes
By the end of this lecture, students should be able to;
•describe case control study design.
•explain the advantages and disadvantages of case
control study design.
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3. Case-control studies
•Purpose is to establish association between exposure to
risk factors and disease.
•Members of the population with the disease are
selected into the study at the outset and risk factor
information is collected retrospectively
–Known as CASES
•A second group of individuals who do not have the
disease is also included in the study
–Known as CONTROLS
•Often used in the study of rare disease or preliminary
study
–Where little is known about the association between the risk
factors and disease of interest

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4. (cont.) Case control studies
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5. (cont.) Case control studies
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6. (cont.) Case control studies
•Case-control studies are prone to bias and confounding
•To minimize bias care must be taken in
–the selection of cases and control
–establishing definitions of disease, risk factors
–ensuring there are no confounding associations between
detection of disease and risk factors exposure.

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7. Choice of cases
•Care must be taken when choosing cases for the study.
•It is important to distinguish between stages or subtypes
of disease and to define a measure of health status
–E.g. when studying physical activity it is important to define what
is meant by physical activity in terms of types, nature and level.
•It is also important to establish whether interest is in
–incident cases, subjects entered into study on detection of
disease, OR
–prevalent cases, those who have been diagnosed as having the
disease prior to the study
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8. (cont.) Choice of cases
•Views, behavior and reports of exposure to risk factor
amongst incident cases and prevalent cases will tend to
differ
–Those diagnosed previously are likely to be more informed about
the disease and may have altered their behavior and attitudes
•Incident case design is preferred as it reduces recall bias
and over-representation of cases with long standing
base.
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9. Choice of controls
•Controls should
–come from the same population at risk of disease
–not have the disease
–be representative of the target population
•Selecting controls often proves harder than cases and
requires great care in the prevention of bias.
•A sampling frame of hospital patients is often used to
select controls
–however risk factors such as diet and smoking are commonly
linked to many diseases
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10. (cont.) Choice of controls
•Selecting controls from hospital patients sampling frame
might therefore over-estimate population exposure
–underestimation of association between disease and exposure
–to overcome this; use more than one control group
•Multiple controls can be used for each case
–Giving the study greater power
–Particularly where the number of cases is small due to the
disease being rare.

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11. Exposure to risk factors and matching
•Exposure measurements are reliant on memory
(interviewed retrospectively) and/or medical records.
•Exposure estimates are vulnerable to recall bias
–Those with disease are more likely to remember exposure than
those without
•Exposure estimates are vulnerable to interview or
measurement bias
–Interviewer interviews or reports findings systematically
differently between cases and control and confounding factors
–Can be overcome by including blinding in the design so that they
do not know who is a case and who is a control at the time of
interview.
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12. (cont.) Exposure to risk factors and matching
•Confounding factors must be identified prior to the start
of study.
•Individuals matched to controls where it is thought that
other factors, aside from those risk factors of interest,
might contribute to the development of disease
–E.g: age, sex,
•The factor upon which cases and controls are matched
can not be studied as risk factor.
•Alternative method of overcoming confounding is to
collect relevant information on them and adjust
statistically.
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13. (cont.) Exposure to risk factors and matching
•Matching is used in case-control studies for;
–To permit allowance for confounders which are complex of
difficult to define
•e.g. by comparing within identical twin case/control pairs it is possible to
allow for ill-defined genetic confounders.
–To make allowance for confounders statistically more efficient.
Efficient analysis requires that there be a similar ratio of cases to
controls at each level of exposure to the confounding variable
–To reduce biases in the ascertainment of exposure.
•Egg. Data collected from deceased cases should be matched to deceased
control as information is collected from their family members.
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14. Analysis of data
•Case control study measure the odd of exposure based
on disease
–Compared to cohort study which measure relative risk of disease
based on exposure
•Odds ratio has two components;
–The odds of exposure for cases
–The odds of exposure for controls
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15. (cont.) Analysis of data




•The odds of exposure for cases
–Number of cases exposed / number of cases unexposed given
by Odds / cases = a / b
•The odds of exposure for controls
–Number of controls exposed / number of controls unexposed
given by Odds / controls = c / d
•The estimated Odds Ratio is then Odds = ad / bc


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16. (cont.) Analysis of data
•The Odds Ratio is interpreted as
–OR < 1; Odds of exposure for cases are less than those for
control. Exposure appears to reduce risk of disease.
–OR = 1; Odds of exposure for cases are the same as those for
control. Exposure does not appear to be a risk factor.
–OR > 1; Odds of exposure for cases are more than those for
control. Exposure appears to increase risk of disease.
•A 95% confidence interval (95% CI) gives an indication
of the confidence we have in the estimated Odds Ratio
–E.g. if the entire 95% CI is above 1, it is concluded that exposure
significantly increases the risk of disease at the 95% level.
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17. (cont.) Analysis of data
•Assessment of whether an observed association is likely
to be directly causal and not the results of unrecognized
confounding depends on;
–The size of the relative risk – higher relative risk are less likely to
be explained by unknown confounders.
–The presence of a dose-response relation – the observation of a
higher risk in subjects with a greater exposure to the risk factor
favors a direct causal relationship.
–The existence of plausible biological mechanism which might
explain a causal relationship.
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18. Advantages of case control studies
•Quick
•Cheap
•Particularly suited to the study of rare diseases as the
diseased are selected at the outset of the study.
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19. Disadvantages of case control studies
•Difficulties in overcoming potential bias and confounding.
•The successful selection of both cases and controls who
are representative of their respective population is often
difficult.
•An inability to infer causality and no information on the
chronology of disease and exposure.
•Inefficient in studying risk factors which are rare.
•Studies are often not population based, therefore it is
impossible to calculate incidence of disease.
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20. Recapitulate
In this lecture, you have been exposed to;
•definition of case-control studies
•choice of cases and controls
•exposure to risk factors and matching
•analysis of data for case-control studies
•advantages and disadvantages of case-control studies
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21. Thank YouThank You
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