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Introduction to Cohort Studies
MalimuKampala international University
Learning Objectives
When you have completed this session you will be able to:
Describe the characteristics of a cohort study List the types of bias most likely to affect a cohort
study List the conditions under which a cohort study is an
appropriate choice to address a research question Describe the advantages and disadvantages of
cohort study
PersonPlace
Time
Cases
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10
0200400600800
10001200
0-4 '5-14 '15-44
'45-64
'64+
Age Group
Descriptive Epidemiology
Who? Where? When?
But sometimes also: Why? What are the risk factors for neonatal
tetanus? What factors are associated with increased
mortality for persons with measles? Does smoking cause lung cancer?
Analytical epidemiology
Analytic Studies
To test whether certain factors are “associated” Association is a statistical concept To look at association we need to move away from
description of a factor in one group We need a “comparison group” Is cancer more common in those exposed to uranium
compared to those who have never being exposed ? Is uranium associated with cancer ? Is this association statistically significant ? Could it have occurred by chance ?
Case-control
Cohort
Individuals
InterventionRetrospective
Prospective
Descriptive
Populations
Analytical
Observational
Case-series
Cross-sectional
Correlational
Clinical trials
Epidemiological studies
Observational Analytic Studies
exposure outcome cohort
case-control
cross-sectional
exposure outcome
exposure outcome
Disease among exposed?
Disease among non-exposed?
Usually prospective
Cohort study
Populationat risk
Exposed
Not Exposed
and
What is a cohort?
Cohort: Latin word for 1 of the 10 divisions of a Roman legion
A group of individuals sharing same experience Followed-up for a specified period of time
Examples birth cohort Workers at a chemical plant KIU first cohort
Design of a Cohort Study
Individuals “choose” theirexposure status
Sub-classifications of Cohort Studies Time perspective
Prospective Retrospective
Population dynamics Closed population Open population
Terminology: Retrospective or Prospective?
Suggest use the terms “retrospective” or “prospective” to refer to the timing of events in relation to initiation of study.
(Hennekens and Buring, 1987)
Retrospective cohort study: exposure and disease have occurred prior to start of study
Prospective cohort study: disease has not occurred prior to start of study
Exposureoccurrence
Study starts Diseaseoccurrence
Prospective Cohort Study
Time
+-
+ -ill
exp+-
exp
Prospective assessment of exposure and disease
Selection of population
Prospective Cohort Study
Chernobyl, Industrial accidents, Flood victims
+-
+ -ill
exp+-
exp
Diseaseoccurrence
Study startsExposureoccurrence
Prospective assessment of disease
Selection based on exposure
Steps in a prospective cohort study
Define the population at risk (=cohort) Determine exposure to a factor of interest of all
subjects in the cohort Follow exposed and non-exposed forward in time to
ascertain whether they develop the outcome of interest Compare the outcomes in the exposed and the
unexposed group with each other
Foodborne outbreaks, closed environment outbreaks (school, prisons, …)
Retrospective Cohort Study
Study takes place
Diseaseoccurrence
Exposureoccurrence
Retrospective assessment of exposure and disease
Selection based on population
+-
+ -ill
exp
Retrospective cohort studies
• Well defined population • Exposed and unexposed can be identified • Outcome (ill or not ill) can be ascertained Opportunity to go back in time, categorise people according to their exposure and then determine their outcome
For example, weddings, parties, hotels, occupational exposures, etc.
Closed and Open Populations
Closed population adds no new members over time loses members only to death
Open population may gain members over time (immigration or
birth) may lose members who are still alive through
emigration
Issues in Design of Cohort StudiesSources of DataExposure Information Pre-existing records
Availability for much of cohort Inexpensive Objective, bias-free categorisation of exposure status But – insufficient detail and no information on potential confounders
Information from study subjects Information on data not routinely collected Questionnaires/interviews Potential bias
Ascertainment of exposure must be comparable for all
Issues in Design of Cohort StudiesSources of DataOutcome Information Obtain complete, comparable, unbiased information Death certificates (potential bias when cause-
specific mortality) Medical records, Medical Aid schemes, etc. From study subjects Periodic direct medical examinations
Apply equally to exposed and non-exposed
Biases in Cohort studies
1. Loss to follow up Failure to ascertain outcome data is the major
source of potential bias Length of follow-up period is related to latency
period of disease The longer the follow-up period the more difficult
to ensure complete data If lost to follow-up is large (eg, 30-40%) ?
Validity ? Loss to follow-up may be differential
2. Participation bias
Agreeing participants may differ from non-participants
This affect external validity more then internal3. Misclassification bias Misclassification due to exposure status is
common Can be random (equally for exposed and
unexposed) or non-random
4. Ascertainment bias
Biases in ascertaining the outcome. Outcomes should be ascertained equally by
exposure status
Distribution of illness according to exposure in a cohort study
Exposed
Not exposed
ILL NOT ILL
a b
c d
a+b
c+d
Incidence
a+b
c+d
a
c
Relative risk = Incidence exposed / Incidence not exposed
Cohort study about bottled water as risk factor for illness
Drink bottled water 40
30
30
Risk
3040
Relative Risk (RR) = 30 / 405 / 30
= 4.4
ILL NOT ILL
10
25 5Do not drinkbottled water
5 30
Interpretation of Relative Risk The risk of illness among those who drink
bottled water is 4.4 times higher than among those who do not drink bottled water.
Advantages of cohort studies Directly measure risk or rate Measures of effect have clear meaning and are
easily understandable Temporal relationship between exposure &
disease can be established Prospective cohort studies less susceptible to
selection bias because outcome not known Well suited to rare exposures Several outcomes can be examined in one study
Disadvantages of cohort studies
Large sample size Latency period Loss to follow up Exposure can change over time Multiple exposures = difficult Cost Time consuming
Recap
Now that you have completed this session you should be able to:
Describe the characteristics of a cohort study List the types of bias most likely to affect a cohort
study List the conditions under which a cohort study is an
appropriate choice to address a research question Describe the advantages and disadvantages of
cohort study versus a case-control study