This document describes a presentation on cohort studies. It defines a cohort study as observing groups of individuals who are alike in many ways but differ in terms of exposure to a certain factor, such as smoking. It outlines the key steps in conducting a cohort study, including selecting study subjects, obtaining exposure data, follow up, and analysis. Analysis involves calculating relative risk to measure the association between exposure and disease outcomes. A relative risk above 1 indicates a positive association, while below 1 suggests a protective association. An example calculates a relative risk of 4 for the association between smoking and lung cancer based on outcomes from a prospective cohort study. Strengths of cohort studies are that temporal relationships can be established and incidence measured, but limitations include being time-

1. 09/12/2019 1Dr Tanveer Rehman PSM JIPMER

2. 09/12/2019 2Dr Tanveer Rehman PSM JIPMER

3. Dr Tanveer Rehman
Junior Resident
PSM, JIPMER

4. Specific learning objectives
At the end of the session, the students should be able to
1. describe the design of a cohort study.
2. list the steps involved in carrying out a cohort study.
3. calculate and interpret relative risk in a cohort study.
4. explain the strengths and limitations of a cohort study.
09/12/2019 4Dr Tanveer Rehman PSM JIPMER

5. Revision – Study Designs
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Did the
investigator
assign
exposure?
Yes
Experimental
study
No
Observational
study
Comparison
group present?
Yes
Analytical
study
Cohort
study
Case
control
study
Cross
sectional
study
No
Descriptive
study
Dr Tanveer Rehman PSM JIPMER

6. 09/12/2019 6Dr Tanveer Rehman PSM JIPMER

7. Cohort
1. Military, not medical, roots: 300–600 soldiers in the
Roman army.
2. A group of people who share a common characteristic
or experience within a defined time period.
3. Birth cohort
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8. Concept
1. Cohorts identified prior to appearance of disease:
exposed and non exposed
2. Study groups observed over a period of time
3. Compare the incidence of disease in the two groups
4. Temporal relationship: exposure preceded the onset of
disease
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9. Types of Cohort Study
1. Prospective cohort study
2. Retrospective cohort study
3. Ambi-directional cohort study
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10. Prospective / Concurrent Cohort Study
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Exposed
Disease No disease
Not Exposed
Disease No disease
2019
2030
Dr Tanveer Rehman PSM JIPMER

11. Retrospective / Historical Cohort Study
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Exposed
Disease No disease
Not Exposed
Disease No disease
2010
2019
Dr Tanveer Rehman PSM JIPMER

12. Ambi-directional Cohort Study
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20302010
2019
Dr Tanveer Rehman PSM JIPMER

13. Steps in a cohort study
1. Selection of study subjects
2. Obtain data on exposure status
3. Follow up
4. Analysis
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14. Selection of study subjects
A. General population for common exposure
B. Special groups:
i. Select groups. e.g.: doctors, teachers
ii. Exposure groups. e.g.: Industrial workers
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15. Obtain data on exposure status
1. Cohort members: personal interviews, mail questionnaires
2. Review of records: details of treatment
3. Medical examination: blood pressure
4. Environmental surveys
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16. Follow up
1. From point of exposure to the development of outcome
of interest
2. Length of follow up: weeks to decades
3. Problems: Incomplete follow up due to withdrawal from
study, death or migration
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17. Analysis of results
1. Form the 2 X 2 table
2. Data analysed in terms of:
a. Incidence of disease in a population – Absolute Risk
b. Association of disease with exposure –
Relative Risk (or also known as Risk Ratio)
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18. Relative Risk
1. Measure of association between exposure to a factor
and development of a disease or outcome of interest
2. Risk Ratio = Risk in Exposed
Risk in Non Exposed
= Incidence of disease in exposed
Incidence of disease in non-exposed
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19. Interpretation of Relative Risk (RR)
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If RR = 1
Risk in exposed equal to risk in non exposed
(No association)
If RR > 1
Risk in exposed greater than risk in non exposed
(positive association - causal)
If RR < 1
Risk in exposed less than risk in non exposed
(negative association - protective)
Dr Tanveer Rehman PSM JIPMER

20. Exercise
1000 smokers and 1000 non-smokers were followed up to
see how many of them develop lung cancer.
After 25 years of follow up, out of 1000 smokers, 200
people had developed lung cancer.
Out of 1000 non-smokers, 50 people had developed lung
cancer.
1. What is the study design employed?
2. Is there any association between smoking & lung
cancer based on these results?
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21. Solution
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200/1000
50/1000
Relative risk = = 4
Dr Tanveer Rehman PSM JIPMER

22. Strengths
1. Temporal relationship can be established
2. Natural history of disease can be identified
3. Incidence can be measured
4. Multiple outcomes can be assessed
5. Rare exposures can be studied
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23. Limitations
1. Unsuitable for rare outcomes
2. Time consuming
3. Costly
4. Loss to follow up
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24. Summary
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Relative Risk= (Incidence among exposed)/(Incidence among non-exposed)
Dr Tanveer Rehman PSM JIPMER

25. THANK YOU
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