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COHORT STUDY

DR DEEPA PRIYA

ASSISTANT PROFESSOR

STATISTICIAN

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CONTENTS

  • DEFINITION OF COHORT
  • DESIGN OF A COHORT STUDY
  • TYPES
  • ELEMENTS IN COHORT STUDY
  • BIAS
  • ADVANTAGES & DISADVANTAGES
  • SUMMARY
  • REFERENCES

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EPIDEMIOLOGICAL STUDIES

Observational (Non-Interventional)

Experimental (Interventional)

Descriptive

Analytical

Cross sectional

Case – Control

COHORT

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Cohors-Refers to group of warriors marching forward together in time

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COHORT STUDY

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  • Observational epidemiological study done for evaluating the association between a suspected cause (exposure) and subsequent risk of developing the disease (outcome) under study.

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CAUSE EFFECT.

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DESIGN OF A COHORT STUDY

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People without disease

Exposed

( Study cohort )

Not exposed

(Control cohort )

Disease

No disease

Disease

No disease

Direction of inquiry

Time

P

O

P

U

L

A

T

I

O

N

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FRAMEWORK ( 2X2 TABLE):�

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Cohort

Diseased

Non-diseased

Total

Exposed

a

b

a + b

Non-exposed

c

d

c + d

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Total

a + c

b + d

a + b + c + d

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TYPES OF COHORT STUDY

  • Prospective cohort study

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  • Retrospective cohort study

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  • Combination of prospective and retrospective cohort design.

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PROSPECTIVE COHORT STUDY

  • A prospective cohort study (or "current" cohort study) is one in which the outcome (e.g., disease) has not yet occurred at the time the investigation begins.
  • Most prospective studies begin in the present and continue into future.
  • For example, the long-term effects of exposure to uranium by assessing subsequent development of lung cancer in both the groups.
  • Smoking and Lung cancer

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RETROSPECTIVE COHORT STUDY

  • “Historical cohort” or “Non-concurrent prospective study”
  • The investigator goes back in time, 10 to 30 years to select his study groups from seeing records of past employment, medical or other records and traces them forward through time, usually up to the present.

E.g. 1. Role of arsenic in human carcinogenesis

2. Study of lung cancer in Uranium miners 3. Polyvinyl chloride & Angiosarcoma of the liver

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ELEMENTS OF COHORT STUDY

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  • Selection of study subjects

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  • Obtaining data on exposure

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  • Selection of comparison group

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  • Follow up

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  • Analysis

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ANALYSIS

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The data should now be analysed to find out;

  • Whether any association exists between the suspected factor and the disease under study

- Incidence rate

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  • If present, estimate the risk (what is the strength of the association)

- Relative risk & Attributable risk

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ANALYSIS FRAMEWORK

Calculation of Incidence

Incidence of disease among

exposed group (study cohort) IE = a /a+b

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Incidence of disease among

non-exposed group INE = c / c+d

(control cohort)

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RELATIVE RISK

Also called as “Risk Ratio”

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Direct measure of “strength of association” between the cause and effect.

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Index for assessing the etiological role of factor in

disease.

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RELATIVE RISK�

Incidence of disease among exposed (IE )

RR = ---------------------------------------------------

Incidence of disease among

non – exposed (I NE )

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�INTERPRETATION OF RELATIVE RISK (RR)

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If RR = 1 - No association

If RR = >1 - Indicates positive association between exposure and disease (risk)

If RR = <1 - Indicates negative association and estimates

Protective effect / benefit

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ATTRIBUTABLE RISK

  • Indicates to what extent the disease under study can be attributed to particular exposure.

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  • Public health importance.

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  • Suggests that the amount of disease that might be eliminated if the factor under study could be controlled or eliminated.

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ATTRIBUTABLE RISK

Attributable Risk percent (AR %)

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IE - INE

AR% = -------------- x 100

IE

In the above Eg: AR% = 10 – 1 /10 x100

= 90%

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TYPES OF BIAS IN COHORT STUDIES

SELECTION BIAS

FOLLOW UP BIAS

MEASUREMENT BIAS

CONFOUNDING BIAS

OBSERVER BIAS

CROSS-OVER BIAS (“HAWTHORNE EFFECT”)

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ADVANTAGES OF COHORT

  • Several possible outcomes related to exposure can be studied simultaneously
  • Relative risk can be directly estimated
  • Dose response ratios can be calculated
  • Bias can be minimized.
  • Incidence can be calculated.
  • Rare exposures can be studied.
  • Several possible outcomes can be studied
  • Study the natural history of the disease.

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DISADVANTAGES OF COHORT

  • Involves a large number of people
  • Long time for completion
  • Loss to Follow up
  • Administrative/ logistic problem
  • Loss of substantial proportion of original cohort
  • Lack of appropriate past records
  • Inefficient for rare diseases.
  • Expensive
  • Changes in diagnostic criteria over time

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SUMMARY

  • CAUSE EFFECT.
  • TYPES: Prospective cohort study, Retrospective cohort study, Combination
  • Elements of cohort study
  • Relative Risk
  • Attributable Risk
  • Types of bias
  • Advantages and disadvantages of cohort study

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REFERENCE

  • Parks textbook of Preventive and Social Medicine 26th edition
  • Roger Detels, James Mc Even -Oxford Text Book of Public Health
  • Col  Rajvir Bhalwar Public Health and Preventive Medicine – “The RED BOOK” Diamond Jubilee Edition New light publishers 2008;p.26-29
  • Leon Gordis. Epidemiology. 5th ed. Elsevier Saunders 2014; p.179-242

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