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CASE CONTROL STUDY

DR. JENIFA

SENIOR RESIDENT

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EPIDEMIOLOGICAL STUDIES

Observational (Non-Interventional)

Experimental (Interventional)

Descriptive

Analytical

Cross sectional

CASE – CONTROL

Cohort

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Introduction�Analytical Studies are

  • 2nd major type of epidemiological studies
  • Objective : Test hypotheses
  • The subject of interest is the individual within the population
  • Inference ---- To the population from where they are selected

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CASE CONTROL STUDY

  • Retrospective study
  • First approach to test causal hypothesis

THREE FEATURES :

  • Both exposure & outcome have occurred
  • Cause Effect
  • Control or comparison group

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CASE-CONTROL STUDY

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BASIC STEPS

1. Selection of cases & controls

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2. Matching

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3. Measurement of exposure

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4. Analysis & Interpretation

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STEP 1- Selection of cases & controls

  • SELECTION OF CASES :

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A) Definition of a case

1. Diagnostic criteria

2. Inclusion & Exclusion Criteria

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B) Sources

1. Hospitals

2. General Population

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CONTD…..

  • SELECTION OF CONTROLS

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1. Free from the disease

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2. Similar to the cases

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3. Most difficult and important

step

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CONTD….

4. Sources

a) Hospital

b) Relatives

c) Neighbourhood

d) General Population

HOW MANY

Large study 1 : 1

Small study 1 : 2 , 3 or 4

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STEP 2 - Matching

“the process by which we select controls in such a way that they are similar to cases with regard to certain selected variables which are known to influence the outcome of disease & which, if not adequately matched for comparability, could distort the results”

  • Types – group or pair

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STEP 3 – Measurement of Exposure

  • Definition & Criteria about exposure

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  • Information - precisely the same

manner

  • Collection - Interviews

- Questionnaires

- Past Records

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STEP 4 – Analysis & Interpretation

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RISK FACTOR(S)

CASES

CONTROLS

PRESENT

a

b

ABSENT

c

d

TOTAL

a + c

b + d

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CONTD….

  • Exposure rates

Cases = a / a + c

Controls = b / b + d

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  • Estimation of risk : Measure of the strength of the association between risk factor and outcome

Odds Ratio = ad / bc

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ODDS RATIO

“it indicates the odds of exposure among the cases divided by the odds of the exposure among controls”

1= No effect

> 1= Increased risk associated

with exposure

< 1= Protective effect

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BIAS

“is any systematic error in the determination of the association between the exposure and disease”

a. Bias due to confounding

b. Recall bias

c. Selection bias

d. Interviewer’s bias

e. Berksonian bias

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PREVENTION OF BIAS

  • Blinding (interviewer’s bias)

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  • Matching (confounding bias)

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  • Multiple sources of cases & controls

(selection bias)

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ADVANTAGES

  • Easy, Rapid, Inexpensive
  • Small sample
  • No risk to subjects
  • Rare diseases
  • Causal pathway may span many decades
  • Several aetiological factors
  • Ethical problem less

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DISADVANTAGES

  • Problem of bias

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  • Selection of appropriate control group

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  • No incidence rate & temporal association

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