5. cohort studies

Cohort Study
Dr Naveen Phuyal
MBBS,MD

Synonyms
• Longitudinal study
• Prospective study
• Incidence study
• Forward-looking study

What is a Cohort?
• Group of people who share a common
characteristic or experience within a defined
time period.
Age
Occupation
Exposure to drug or vaccine
Pregnancy
Insured person

• An ancient Roman military unit, comprising six
centuries, equal to one tenth of a legion.
• A group of people banded together or treated
as a group.
What is a Cohort?

When is a cohort study warranted?
• Good evidence of association between exposure
and outcome as shown by descriptive and case
control studies
• When exposure is rare but incidence is high
among exposed
• When attrition of people can be minimized
• When ample of funds are available

Research
methods
Observational
Descriptive
Case series,
case reports,
CS, cohort
Analytical
Ecological Cross-
sectional
Case control Cohort
Experimental
Controlled
Uncontrolled,
Non-random

Position in the evidence hierarchy
7

Features of Cohort study
• Cohorts are identified prior to appearance of
disease under investigation
• Study groups are observed over a period of
time to determine the frequency of disease
among them
• Study proceeds forward from cause to effect

• Key element of Cohort study is time.
• Following the exposed and unexposed groups over
time cohort studies are uniquely equipped to
describe process and mechanisms by which
exposures relate to the development of disease.
• Cohort study is the primary tool to study time and
medicine ( Samet 2000)

• They provide the data to describe when disease
occur and track their consequences over time.
• Factors that cause disease or early signs of
disease can be monitored over time.
• Diversity of individuals in cohort study: provides
data to identify risk factors that make certain
individual more susceptible.

• Data collected in cohort studies are useful to
describe the prognostic markers of exposure.
• Multisite cohort studies may serve additional role
of characterizing where disease occur and to
what extent the diseases are spread in different
locations.
• A cohort study with adequate sample and follow
up enables us to understand natural history of
disease.

• Once a measure of frequency of disease occurrence
(incident cases in person-years) is adopted, cohort
studies allow the direct comparison of the risk of
becoming ill in several groups.
• This comparison can be relative or absolute
– Relative: how many times higher or lower is the risk
between exposed and unexposed (relative risk)
– Absolute: how much difference in risk is there between
exposed and unexposed ( attributable risk)

• Relative risk will give you causal relationship
between disease and exposure.
• Attributable risk measures the change of
incidence due to exposure in question ( it
quantifies the burden of disease that an exposure
exerts in a population.)
• Identification of exposures and risk factors for a
disease forms the basis for prevention.

Cohort based associations that have resulted in
prevention strategies
• Lung Ca and smoking: avoid cigarette
• Unprotected sex and HIV: avoid
• IV drug use and HIV: avoid
• To prevent heart disease: avoid high LDL and
low HDL levels
• To prevent Cervical cancer: avoid Papilloma
infection

General consideration
• Cohorts must be free from disease under
study.
• Both the study and control groups should be
equally susceptible to the disease under study
• Both groups should be comparable in respect
to all possible variables
• Diagnostic and eligibility criteria of disease
must be defined beforehand.

Types of cohort studies
1. Prospective cohort studies
2. Retrospective cohort studies
3. Combination of retrospective and
prospective cohort studies

Retrospective plus prospective cohort
study

Elements of cohort study
• Selection of study subjects
• Obtaining data on exposure
• Selection of comparison groups
• Follow up
• Analysis

1. Selection of study subjects
1. General population
when exposure (cause) of death is frequent in general
population, residing in well defined geographical, political and
administrative areas.
Eg. Framingham heart study
2. Special groups
Select groups: doctors, nurses, lawyers, teachers
college alumni, employees, volunteers.
Exposure groups: special exposure to physical, chemical
and other agents.

1. Selection of study subjects

2. Obtaining data in exposure
• Cohort members: interviews/ questionnaires
• Review of records: dose of radiation/ kinds of
surgery/ details of medical treatment
• Medical examination or special tests: blood
pressure, serum cholesterol, ECG
• Environmental surveys: where the cohort lived or
worked

• Information about exposure will allow
classification of cohort members:
1. According to whether or not they have
exposed to suspected factor
2. According to degree of exposure at least in
broad class in case of special exposure
groups
2. Obtaining data in exposure

3. Selection of comparison groups
i. Internal comparisons
i. External comparisons
i. Comparison with general population rates

Internal comparison
• No outside comparison group required.
• A single cohort enters the study and on basis
of information obtained, classified into several
comparison groups according to degree or
levels of exposure.

External comparison
• When information on degree of exposure is not
available
• Eg. Smokers and non-smokers
• Cohort of radiologists with cohort of opthalmologists
• The study and control cohorts should be similar in
demographic and possibly important variables other
than those under study.

Comparison with general population rates
• Frequency of lung cancer among uranium
mine workers vs frequency of lung cancer in
general population
• Asbestos workers vs general population
cancers

4. Follow up
• Periodic medical examinations
• Review of records: physician/ hospital
• Routine surveillance of death records
• Mailed questionnaires, phone calls, periodic
home visits- preferably all three every year
• Loss to follow up occurs due to death, change of
residence, migration, withdrawal of occupation
• Achieve 95% follow up as much as possible

5. Analysis
1. Incidence rates
1. Among exposed
2. Among not exposed
2. Estimation of risk
1. Relative risk
1. Attributable risk
2. Population attributable risk

Disease
Exposure Yes No Total
Yes a b a+b
No c d c+d
Total a+c b+d a+b+c+d
Incidence rates among exposed= (a/a+b)*1000
Incidence rates among non-exposed= (c/c+d)*1000

Relative risk
Disease
Exposure Yes No Total
Yes a b a+b
No c d c+d
Total a+c b+d a+b+c+d
Relative risk is the ratio of incidence among exposed to incidence among non-exposed
RR=( a/a+b)/( c/c+d)

Relative risk
• Direct measure of strength of association
between suspected cause and effect
• RR=1 (no association)
• RR>1 (positive association)
• If RR=2 ( incidence rate of disease is 2 times
higher among exposed than non exposed or
100% increase in risk)
Cigarette
smoking
Lung Ca
developed
No Lung Ca
devloped
Total
Yes 70 6930 7000
No 3 2997 3000
RR= (70/7000)/ (3/3000)= 10
Smokers are 10 times at greater risk
Of
devloping lung Ca than non smokers

Attributable risk
• Also called as ‘ risk difference’
• AR is the difference in incidence rates of disease
(or death) between an exposed group and non-
exposed group.
• AR= Incidence of disease rate among exposed
– incidence of disease rate among non-exposed

Attributable risk percentage
AR%= Incidence of disease rate among exposed
– incidence of disease rate among non-exposed
incidence of disease among exposed
In above example AR= (10-1) X 100 = 90%
10
90 percent of lung cancer among smokers was due to smoking.
This is the amount of disease that might be eliminated if the
factor under study is controlled or eliminated.
X 100

Population attributable risk
PAR= It – INE
PAR %= It – INE
It
PAR % provides the estimate of the amount by
which the disease could be reduced in that
population if the suspected factor was
eliminated or modified.
X 100

RR vs AR
• The size of RR is a better index than AR for
assessing the etiological role of a factor in
disease
• Larger the RR, stronger the association between
cause and effect
• AR gives better idea on impact of successful
preventive or public health programme might
have in reducing the problem

Advantages of cohort studies
• Incidence can be calculated.
• Several outcomes can be studied simultaneously.
• Provide direct estimate of relative risk.
• Dose response ratios can be calculated.
• Certain bias can be minimised.

Disadvantages of cohort studies
• Involves large no. of people and unsuitable for
study of diseases with low incidence.
• Takes long time to conduct the study.
• Loss of staff, loss of fund.
• Requires extensive record keeping.
• Loss to follow up.
• Selection of comparison group is difficult.
• Expensive
• Study may alter the participants behavior.
• Ethical problems.

Case control
• Effect to cause
• Starts with disease
• Tests weather suspected
cause is more frequent
among diseased or
undiseased.
• Fewer subjects
• Quick results
• Suitable for rare disease
• OR
• Relatively cheap
Cohort
• Cause to effect
• Starts with exposure
• Tests weather suspected
disease occurs more
frequenty among exposed
or not exposed
• Large no. of subjects
• Long follow up
• Suitable for rare exposures
• RR/ AR
• Expensive

References
• Epidemiology, Fifth edition. Leon Gordis
• Park.s textbook of Prevntive and social
medicine, 23rd edition. K. Park
• Maxcy-Roseneu-Last, Public Health and
Preventive medicine, 15th edition. Robert B
Wallace.
• Oxford textbook of Public Health,Fifth edition

5. cohort studies

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