II MBBS class Cohort study

1. Cohort study
Dr. Rizwan S A, M.D.,
Assistant Professor,
Department of Community Medicine,
VMCH&RI, Madurai.
10.11.2014
II MBBS, Epidemiology series 1

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
II MBBS, Epidemiology series 2

3. Procedures in descriptive epidemiology
1. Define the population
2. Define and describe the disease
3. Measure the disease
4. Compare
5. Formulate hypothesis
II MBBS, Epidemiology series 3

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
II MBBS, Epidemiology series 4

5. Hills criteria
1. Temporality
2. Strength
3. Specificity
4. Consistency
5. Biological plausibility
6. Coherence
II MBBS, Epidemiology series 5

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
II MBBS, Epidemiology series 6

7. Dogma of cohort study
Healthy people Exposure occurs Exposed &
unexposed
Disease
occurs
Diseased &
non-diseased
II MBBS, Epidemiology series 7

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
II MBBS, Epidemiology series 8

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
II MBBS, Epidemiology series 9

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
II MBBS, Epidemiology series 10

11. Types of cohort study
1. Prospective
2. Retrospective
3. Combined - Amphi
Nov 2013 Nov 2014 Nov 2015
Past Present Future
II MBBS, Epidemiology series 11

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
II MBBS, Epidemiology series 12

13. 1. Selection of subjects
• General population
• Special groups
II MBBS, Epidemiology series 13

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
II MBBS, Epidemiology series 14

15. 3. Selection of comparison
• Internal comparison
• External comparison
• General population
II MBBS, Epidemiology series 15

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%
II MBBS, Epidemiology series 16

17. 5. Analysis
• Incidence of disease in exposed =
• Incidence of disease in non-exposed =
• Relative risk (RR) =
II MBBS, Epidemiology series 17

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
II MBBS, Epidemiology series 18

19. Attributable risk
II MBBS, Epidemiology series 19

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%
II MBBS, Epidemiology series 20

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%
II MBBS, Epidemiology series 21

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%
II MBBS, Epidemiology series 22

23. Advantages
• Incidence and RR can be calculated
• One exposure and multiple outcomes
• Dose response ratios
• Recall bias reduced
II MBBS, Epidemiology series 23

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
II MBBS, Epidemiology series 24

25. Examples of famous cohort studies
• British doctors study on smoking and lung
cancer
• The Framingham heart study
• Oral contraceptives study
II MBBS, Epidemiology series 25

26. Email your doubts to: sarizwan1986@outlook.com
You can download these slides at http://www.slideshare.net/RizwanSa
THANKS FOR LISTENING
II MBBS, Epidemiology series 26