Research Methodology‘Case control study’

Dr. Rizwan S A, M.D.,Assistant Professor,

Department of Community Medicine,VMCH&RI, Madurai.

17.11.2014

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Classification of research methods

Research methods

Observational

Descriptive

Case series, case reports,

CS, cohort

Analytical

Ecological Cross-sectional

Case control Cohort

Experimental

ControlledUncontrolled,

Non-random

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Procedures in descriptive epidemiology

1. Define the population

2. Define and describe the disease

3. Measure the disease

4. Compare

5. Formulate hypothesis

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Association

• Defined as the co-occurrence of two or morevariables at a frequency which is more thanthat expected by chance

• Association does not mean causation

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Causation - Hills criteria

1. Temporality

2. Strength

3. Specificity

4. Consistency

5. Biological plausibility

6. Coherence

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Dogma of cohort study

Healthy people Exposure occurs Exposed & unexposed

Disease occurs

Diseased & non-diseased

6

Time

Direction of enquiry II MBBS, Epidemiology series

CASE CONTROL STUDY

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Position in the evidence hierarchy

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Dogma of case control study

Assemble cases –diseased

Time

Direction of enquiry

Assemble controls –not having disease

Measure exposure status

Exposed and non-exposed

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Introduction

• Synonyms – retrospective study

• A study that compares two groups of people: thosewith the disease or condition under study (cases) and avery similar group of people who do not have thedisease or condition (controls).

• Essential elements– Both exposure and disease have occurred– Proceeds from effect to cause– Uses a comparison ‘control’ group

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2 by 2 table

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Diseased- Cases

Non-diseased– Controls

Total

Exposed A B A+B

Non-exposed C D C+D

Total A+C B+D A+B+C+D

Steps in case control study

1. Selection of cases and controls

2. Matching

3. Measurement of exposure

4. Analysis and interpretation

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1. Selection of cases and controls

• Define case – diagnostic and eligibility criteria

• Source of cases – hospital, general population

• Define control – free from disease

• Source of controls – hospital, relatives, neighbourhood controls, general population

• Number of controls

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2. Matching

• Ensure comparability between cases and controls

• A confounding factor is defined as one which isassociated both with exposure an disease, and isdistributed unequally in study and control groups.

• Examples:

• Suspected aetiological factor should not be matched

• Types of matching: pair, frequency

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2. Matching

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3. Measure the exposure

• Define exposure

• Measure in precisely the same manner both for cases and controls

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4. Analyse

• Exposure rates among cases and controls

• Odds Ratio (Cross-product ratio)

– AD/BC

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4. Analysis – Example

• E.g. 1. Depression and Vegetable eating

• Odds of exposure among cases: a/c = 90/130 = 0.6923

• Odds of exposure among controls: b/d = 90/130 = 0.6923

• Odds ratio = 0.6923/0.6923 = 1.0

Individuals WithDepression(Cases)

Individuals WithoutDepression(Controls)

Total

Eat Vegetables

90 90 180

Do Not EatVegetables

130 130 260

Total 220 220 440

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Biases

• Bias due to confounding

• Memory or recall bias

• Selection bias

• Berkesonian bias

• Interviewer bias

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Pros & Cons

Advantages Disadvantages

Easy to carry out Subject to several biases

Rapid results Selection of controls difficult

Inexpensive Incidence can’t be measured

Suitable for rare diseases Association doesn’t mean causation

No risk to subjects Not practical for rare exposure

Minimal attrition

Multiple exposures can be

studiedII MBBS, Epidemiology series 21

Case controls studies Cohort studies

Direction of inquiry from effect to

cause

Direction of inquiry from cause to

effect

Starts with the diseaseStarts with people exposed to the

risk factor or suspected cause

Usually the 1st approach to the

testing of hypothesis, but also useful

for exploratory studies

Reserved for the testing of

precisely formulated hypothesis

Fewer study subjects Larger number of subjects

Quick Long follow-up, delayed results

Suitable for rare diseasesInappropriate when disease or

exposure under investigation is rare

Generally, yields only Odds ratioYields incidence rates, relative risk,

attributable risk

Cannot yield information about

disease other than that under study

Can give information about more

than one disease outcome

Inexpensive ExpensiveII MBBS, Epidemiology series 22

Examples

• Adenocarcinoma of vagina and DES

• OCP and thrombosis

• Thalidomide tragedy

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THANKS FOR LISTENING

Email your doubts to: sarizwan1986@outlook.com

You can download these slides at http://www.slideshare.net/RizwanSa

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