Case Control StudyManish Chaudhary

BPH, MPHmanish264@gmail.com

Introduction

• The case-control study is an analytic epidemiologic research design in which the study population consists of groups who either have (cases) or do not have a particular health problem or outcome (controls).

• The investigator looks back in time to

measure exposure of the study subjects. The exposure is then compared among cases and controls to determine if the exposure could account for the health condition of the cases.

• Hallmark of the case-control study is that it begins with people with the disease (cases) and compares them to people without the disease (controls).

• Also called:- Case-Referent, Case-Compeer, Retrospective Study

• Determines the strength of the association between each predictor variable and the presence or absence of disease.

Introduction Contd…..

Three Distinct FeaturesA) both exposure and outcome have

occurred before the start of the study.B) The study proceeds backwards from

effect to cause and C) It uses a control or comparison group to

support or refute an inference

Characteristics

Steps of Case Control Study

1. Selection of Cases and Controls2. Matching3. Measure of Exposure4. Analysis and Interpretation

A] Definition of Case:-i) Diagnostic Criteria: Must be specified

before the study is undertaken.ii) Eligibility Criteria: Incident cases are

eligible than prevalent cases.B] Sources of Cases:-iii) Hospital patients, patients in

physicians practices, or clinic patients. iv) General Population

1. Selection of Cases

Controls must be similar to the cases as possible, except for the absence of the disease under study.

Sources of Controls:- Should come from

population at risk of disease.• Population of defined area• Hospital patients• Neighbors• Friends • Siblings, spouses or other relatives

1. Selection of Controls

Qualities of Control

• Comparability is more important than representativeness in the selection of controls

• The control must be at risk of getting the disease.

• 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.

• Obtain power by matching more than one (generally < 4) control per case.

• The usual case control ratio is 1:1. • Increasing the ratio of controls to cases increases the

precision and efficiency of the analysis but it also increases the cost to undertake the study.

• Therefore, 1:4 ratio of case and control is suitable.

C o s t

P r ec is io n

N o . o f c o n tr o ls

No.

of c

ases

4

1

Selection of Case and Control

Total Population

Control

Cases

• Process of selecting controls so that they are similar to cases on certain specific characteristics such as sex, age, occupation, body mass index, smoking status and marital status etc.

• The cases and control should be comparable with

confounding factors.

• Methods of matching- group matching, pairing

2. Matching

Potential problems of matching

• It will be difficult to find controls if too many variables are selected for matching.

• Variables used for matching can not be studied as exposures or confounders.

Matching Contd……

3. Measurement of Exposure

• Information about exposure may be obtained by interviews, by questionnaires or by studying past records of cases such as hospitals records, employment records etc.

• Information should be obtained in precisely the same manner for cases and controls.

4. Analysis

Estimation of disease risk associated withexposure (Odds Ratio):-• The ratio of the ratio of exposure in Cases

compared to the ratio of exposure in Control.• Measures the strength of the association

between risk factor and outcome.

Cases Controls

Exposed a bUnexposed c d

OR = ad/bc

Example of Case Control Study

We have sampled 40 Lung Cancer patients admitted in BPKIHS Hospital and 50 controls without Lung Cancer and ask about their smoking habit in the past.

 

Smoker

Non-smoker

 Cases

15(a)

25(b)

Control

5 (c)

45(d)

 

50

40

Odds Ratio (OR) = ad/bc = 5.4

Interpretation:- Smokers showed the risk ofhaving lung cancer 5.4 times that of non

smokers.

Bias in Case Control Study

Bias is the Systematic error in the determination of the association between the exposure and disease

1. Bias due to Confounding2. Memory or Recall Bias3. Selection Bias4. Berkesonian Bias: Arises due to different

rates of admission to hospitals for people with different diseases (i.e. hospital cases and control)

5. Interviewer’s Bias

Important Discoveries by Case Control Study

1950's:• Cigarette smoking and lung cancer1970's• Diethyl stilbestrol and vaginal

adenocarcinoma• Post-menopausal estrogens and endometrial

cancer 1980's• Aspirin and Reyes syndrome• Tampon use and toxic shock syndrome• L-tryptophan and eosinophilia-myalgia

syndrome• AIDS and sexual practices

Advantages

• Quick and inexpensive.• Suitable to investigate rare diseases. • Well suited to the evaluation of outcomes with long

latent periods.• It can examine multiple etiologic factors for a single

disease.• No attrition problem because it doesn't require follow

up. • Ethical problems are minimal.

DISADVANTAGES

• Cannot measure;Incidence, Prevalence and Relative Risk

• Can only study one outcome• Limitations in recall and recall bias• Problem of selection of appropriate

control groups• If disease is relatively common (> 5 to

10%), OR may not be reliable estimate of RR

• Other possible effects of exposure can not be studied

Summary

• Imagine a source population– Consists of exposed and unexposed

people– Gives rise to cases

• Control group is a sample from this source population– Independent of exposure status– Same distribution of exposed persons

(person-time) as in source population

• Determine exposure status of cases and controls

• Calculate odds ratio

Thank You!!!