cohort study - basics
TRANSCRIPT
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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
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Classification of research methods
Research methods
Observational
Descriptive
Case series, case reports,
CS, cohort
Analytical
Ecological Cross-sectional
Cohort Case control
Experimental
Controlled Uncontrolled
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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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Association
• Defined as the co-occurrence of two or morevariables at a frequency which is more thanthat expected by chance
• Association does not mean causation
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Hills criteria
1. Temporality
2. Strength
3. Specificity
4. Consistency
5. Biological plausibility
6. Coherence
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Introduction
• Synonyms – prospective, forward looking,longitudinal, incidence
• Features – healthy people, follow up, cause toeffect
• Cohort – a group of people with commoncharacteristics
• E.g., birth cohort, marriage cohort
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Dogma of cohort study
Healthy people Exposure occurs Exposed & unexposed
Disease occurs
Diseased & non-diseased
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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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Indications for cohort
• Good evidence of association
• Exposure is rare but incidence amongexposure is common
• When loss to follow up can be minimised
• Funds are ample
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General considerations
• Cohort must be free from disease underconsideration
• Both exposed and non-exposed groups shouldbe equally susceptible to disease
• Both exposed and non-exposed groups shouldbe comparable
• Eligibility criteria should be definedbeforehand
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Types of cohort study
Nov 2013 Nov 2014 Nov 2015
Past Present Future
1. Prospective
2. Retrospective
3. Combined - Amphi
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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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1. Selection of subjects
• General population
• Special groups
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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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3. Selection of comparison
• Internal comparison
• External comparison
• General population
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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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5. Analysis
• Incidence of disease in exposed =
• Incidence of disease in non-exposed =
• Relative risk (RR) =
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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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Attributable risk
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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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Example of calculations
• 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%
Lung cancer Normal Total
Smoker 70 6930
Non-smoker 3 2997
Total
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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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Advantages
• Incidence and RR can be calculated
• One exposure and multiple outcomes
• Dose response ratios
• Recall bias reduced
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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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Examples of famous cohort studies
• British doctors study on smoking and lung cancer
• The Framingham heart study
• Oral contraceptives study
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THANKS FOR LISTENING
Email your doubts to: [email protected]
You can download these slides at http://www.slideshare.net/RizwanSa
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