cohort study - basics

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

Classification of research methods

Research methods

Observational

Descriptive

Case series, case reports,

CS, cohort

Analytical

Ecological Cross-sectional

Cohort Case control

Experimental

Controlled Uncontrolled


Procedures in descriptive epidemiology

1. Define the population

2. Define and describe the disease

3. Measure the disease

4. Compare

5. Formulate hypothesis


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


Hills criteria

1. Temporality

2. Strength

3. Specificity

4. Consistency

5. Biological plausibility

6. Coherence


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


Dogma of cohort study

Healthy people Exposure occurs Exposed & unexposed

Disease occurs

Diseased & non-diseased


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


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


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


Types of cohort study

Nov 2013 Nov 2014 Nov 2015

Past Present Future

1. Prospective

2. Retrospective

3. Combined - Amphi


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


1. Selection of subjects

• General population

• Special groups


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


3. Selection of comparison

• Internal comparison

• External comparison

• General population


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%


5. Analysis

• Incidence of disease in exposed =

• Incidence of disease in non-exposed =

• Relative risk (RR) =


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


Attributable risk


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%


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


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%


Advantages

• Incidence and RR can be calculated

• One exposure and multiple outcomes

• Dose response ratios

• Recall bias reduced


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


Examples of famous cohort studies

• British doctors study on smoking and lung cancer

• The Framingham heart study

• Oral contraceptives study


THANKS FOR LISTENING

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cohort study - basics

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