This document contains 26 slides presented by Dr. Rizwan S A on cohort studies. It defines cohort studies as prospective longitudinal studies that follow healthy populations over time to determine the causes of diseases. Key aspects covered include classifying cohort studies as prospective, retrospective or combined; describing the elements of cohort studies such as selecting and following subjects, measuring exposure and outcomes, and analyzing results using measures like relative risk, risk difference and attributable risk. Examples of famous cohort studies on smoking, heart disease and oral contraceptives are also provided.

1. Cohort study
Dr. Rizwan S A, M.D.,
Assistant Professor,
Department of Community Medicine,
VMCH&RI, Madurai.
10.11.2014
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2. Classification of research methods
Research
methods
Observational
Descriptive
Case series,
case reports,
CS, cohort
Analytical
Ecological Cross-sectional
Experimental
Controlled Uncontrolled
Cohort Case control
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3. 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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4. Association
• Defined as the co-occurrence of two or more
variables at a frequency which is more than
that expected by chance
• Association does not mean causation
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5. Hills criteria
1. Temporality
2. Strength
3. Specificity
4. Consistency
5. Biological plausibility
6. Coherence
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6. Introduction
• Synonyms – prospective, forward looking,
longitudinal, incidence
• Features – healthy people, follow up, cause to
effect
• Cohort – a group of people with common
characteristics
• E.g., birth cohort, marriage cohort
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7. Dogma of cohort study
Healthy people Exposure occurs Exposed &
unexposed
Disease
occurs
Diseased &
non-diseased
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8. A 2 by 2 table
Diseased Non-diseased Total
Exposed A B A+B
Non-exposed C D C+D
Total A+C B+D A+B+C+D
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9. Indications for cohort
• Good evidence of association
• Exposure is rare but incidence among
exposure is common
• When loss to follow up can be minimised
• Funds are ample
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10. General considerations
• Cohort must be free from disease under
consideration
• Both exposed and non-exposed groups should
be equally susceptible to disease
• Both exposed and non-exposed groups should
be comparable
• Eligibility criteria should be defined
beforehand
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11. Types of cohort study
1. Prospective
2. Retrospective
3. Combined - Amphi
Nov 2013 Nov 2014 Nov 2015
Past Present Future
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12. Elements of a cohort study
1. Selection of subjects
2. Obtain data on exposure
3. Selection of comparison
4. Follow up and measure outcome
5. Analysis
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13. 1. Selection of subjects
• General population
• Special groups
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14. 2. Obtain data on exposure
• Clearly define exposure
• How?
– Direct interview
– Medical examination
– Record review
– Environmental survey
• Classify into exposed and non-exposed
• Among exposed degree of exposure
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15. 3. Selection of comparison
• Internal comparison
• External comparison
• General population
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16. 4. Follow up and measure outcome
• Clearly define outcome
• Periodic interview or examination
• Loss to follow up
– Denial of consent
– Death
– Migration
• Ideal follow up is >95%
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17. 5. Analysis
• Incidence of disease in exposed =
• Incidence of disease in non-exposed =
• Relative risk (RR) =
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18. Measures of association
• Relative risk (RR) = I (e) / I (ue)
• Risk difference = I (e) - I (ue)
• Attributable risk = [I (e) – I (ue)]/ I (e)
• Population attributable risk
= Pe (RR-1) / Pe (RR-1) + 1
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19. Attributable risk
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20. Fraction, proportion & percentage
Fraction Proportion Percentage
1/3 0.33 33%
2/3 0.66 66%
3/4 0.75 75%
1/4 0.25 25%
2/4 0.50 50%
2/5 0.40 40%
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21. Example of calculations
Lung cancer Normal Total
Smoker 70 6930
Non-smoker 3 2997
Total
• Incidence of disease in exposed = 0.01 or 1%
• Incidence of disease in non-exposed = 0.001 or 0.1%
• Relative risk = 10
• Risk difference = 0.009 or 0.9%
• Attributable risk = 0.9 or 90%
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22. Example of calculations
• Population attributable risk (PAR) = Pe (RR-1) / Pe (RR-1) + 1
• Pe = Prevalence of exposure in the population = 20% of the
population smoke
• PAR = 0.20 (10) / 0.20 (10) + 1
= 2/2+1
= 2/3
= 0.66 or 66%
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23. Advantages
• Incidence and RR can be calculated
• One exposure and multiple outcomes
• Dose response ratios
• Recall bias reduced
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24. Disadvantages
• Unsuitable for rare outcomes
• Long duration
• Administrative problems
• Loss to follow up
• Selection of representative groups
• Diagnostic criteria may change over time
• Expensive
• People may alter their behaviour
• Ethical problems
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25. Examples of famous cohort studies
• British doctors study on smoking and lung
cancer
• The Framingham heart study
• Oral contraceptives study
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26. Email your doubts to: sarizwan1986@outlook.com
You can download these slides at http://www.slideshare.net/RizwanSa
THANKS FOR LISTENING
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