Introduction to Case Control Studies

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Malimu MSc.Epidemiology PhD. Candi.

Department of Epidemiology and Biostatistics, MUHAS./KIU

Learning ObjectivesWhen you have completed this session you will be able to: Describe the characteristics of a case control study List the type of bias most likely to affect a case control study List the conditions under which a case control study is an appropriate

choice to address a research question Define the term “control group” and list the characteristics of a good

control group

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Case-control

Cohort

Individuals

InterventionRetrospective

Prospective

Descriptive

Populations

Analytical

Observational

Case-series

Cross-sectional

Ecologic

Clinical trials

Epidemiological studies

Intro to Case-control studies Case-control studies provide insight on the aetiology of many

condition Prone for bias in selecting cases/controls, eliciting exposure

status and sophisticated nature of its analysis Hallmark of a professional epidemiologist

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Disease

No disease

Exposure

?

?

Retrospective Nature

Case-Control Study

(Case)

(Control)

Design of a Case-control Study

Variants of case-control design Case- control study (Classical) Case-cohort studies Case-only studies Case-crossover studies

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Major Steps in case-control study

Define and select cases Select controls Ascertain exposures Compare exposure in cases and

controls proportions/odds ratios ....

Test any differences for statistical significance

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Observation

Start with cases Are any observed exposures higher

than expected ? To find this out we need a comparison

group This group are known as controls

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Selection of cases Cases should be selected independent of the exposure Not necessarily represent all people with the disease but

controls should be from the same population as the cases Incident cases better than prevalent cases

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Who is the Right Control? As similar to a case as possible but without the disease in

question Selected from the same population or study base as cases Must have the same opportunity for exposure as a case

Must be subject to the same inclusion and exclusion criteria

No one control group is optimal for all situations

Examples of Controls

Population control

Neighbourhood

Hospitals /clinic based-control

Friends

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General Population Controls

Advantages Exposure in the control estimate that of the population Direct calculation of risk Inferences are easy Include health people

Disadvantages Cost Sampling frame

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Neighborhood Controls Advantages:

Inexpensive,efficient Matched for potentially confounding variables

Disadvantages Exposure related to neighborhood

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Hospital Controls Advantages:

Convenient Easily identified More likely to comply, interviewed and tested Same selection procedure as cases

Disadvantages:-Not source population for the cases (Berksons bias)-May have diagnosis interfering with outcome-Generalizability problems

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Friend Controls Advantages

Convenient Include health and cooperative people

Disadvantages Friends may share same exposure (over-matching)-Overlapping of friendships

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Number of controls

Availability

Ratio controls / cases

Trade-off: cost vs. power

Decision based on power calculation

More than one control group?

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Biases in Case-control studies Selection biases

Cases selection –external validity Controls selection-internal validity

Measurement biases Observer bias-favour cases Recall bias-cases remembers more Information bias –equal collection methods

Confounders

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Matching in Case –control studies It is a strategy for controlling potential confounders. Each case is matched with a control in a number of variables E.g. Age, sex, education… Advantages

Help to control un-measurable confounders eg. Genetic-using siblings Eliminate the need to list all possible controls Increase precision of odds ratio

by weakening the assoc. btwn confounder and outcome

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Comparison of cases and control Comparison of occurrence of disease/event is made between

the exposed and unexposed Odds ratio (OR) is the measure of effect OR= odds of exposure among the cases

odds of exposure among the controls

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Cases Controls

Exposed a b

Not exposed c d

Total a + c b + d

Odds ratio = (a/c)/ (b/d) = a x d b x c

Distribution of cases and controls according to exposure in a case control study

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Oral Myocardialcontraceptives Infarction Controls

Yes 693 320

No 307 680

Total 1000 1000

Odds ratio = 693 x680 = 4.79320 x 307

Distribution of myocardial infarction cases and controls by oral contraceptive use

Odds Ratio: Interpretation OR > 1 - the risk of disease in the exposed

group is greater than the risk in the unexposed group

OR = 1 - the risk of disease is the same in the exposed and unexposed (no association)

OR < 1 - the risk of disease in the exposed group is less than the risk in the unexposed

Advantages of Case Control Studies

Rare diseases Multiple exposures Diseases with long latent periods Small sample size

Low cost

Secondary data analysis possible

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Disadvantages of Case-control Studies

Limited to one outcome variable Selective survival effect Selection of controls difficult Not suitable for rare exposures Nonrepresenativeness of cases (Berkson’s

fallacy)!!!! Problems with recall (information bias)

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