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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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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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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