2. 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
4. 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
5. 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 ?
9. 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
10. Design of a Cohort Study
Individuals
“choose” their
exposure status
11. Sub-classifications of Cohort Studies
Time perspective
Prospective
Retrospective
Population dynamics
Closed population
Open population
12. 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
14. Prospective Cohort Study
Chernobyl, Industrial accidents, Flood victims
+
-
+ -
ill
exp
+
-
exp
Disease
occurrence
Study startsExposure
occurrence
Prospective assessment
of disease
Selection based
on exposure
15. 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
16. Foodborne outbreaks, closed environment outbreaks (school, prisons,
…)
Retrospective Cohort Study
Study
takes place
Disease
occurrence
Exposure
occurrence
Retrospective assessment
of exposure and disease
Selection based
on population
+
-
+ -
ill
exp
17. 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.
18. 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
19. Issues in Design of Cohort Studies
Sources of Data
Exposure 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
20. Issues in Design of Cohort Studies
Sources of Data
Outcome 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
21. 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
22. 2. Participation bias
Agreeing participants may differ from non-
participants
This affect external validity more then internal
3. Misclassification bias
Misclassification due to exposure status is
common
Can be random (equally for exposed and
unexposed) or non-random
23. 4. Ascertainment bias
Biases in ascertaining the outcome.
Outcomes should be ascertained equally by
exposure status
24. 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
25. Cohort study about bottled water as
risk factor for illness
Drink bottled
water
40
30
30
Risk
30
40
Relative Risk (RR) = 30 /
405 /
30
=
4.4
ILL NOT ILL
10
255
Do not drink
bottled water
5
30
26. 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.
27. 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
28. Disadvantages of cohort studies
Large sample size
Latency period
Loss to follow up
Exposure can change over time
Multiple exposures = difficult
Cost
Time consuming
29. 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
Notas do Editor
Please do not change the learning objectives without notifying the team.
Move any learning objectives that you don’t expect to cover in class to the “What’s next” slide at the end of the presentation.
For ascertainment can use physical examination, proxy measures ( job title, distance from source) direct measurement (water, air pollution) but that is a problem if exposure occurred before study began. Consider need for baseline assessment and periodic reassessment as it may change over time.
Discuss problems with lost to follow-up and how those lost may differ in important ways from those who are not lost.
Please do not change the learning objectives without notifying the team.
Move any learning objectives that you don’t expect to cover in class to the “What’s next” slide at the end of the presentation.