Cohort study

Dr. Rizwan S A, M.D.,Assistant Professor,

Department of Community Medicine,VMCH&RI, Madurai.

10.11.2014

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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: sarizwan1986@outlook.com

You can download these slides at http://www.slideshare.net/RizwanSa

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