A cohort study involves observing a group of individuals over time to examine exposure-outcome relationships. Key characteristics include prospectively following exposed and unexposed groups to compare disease outcomes. Major biases include loss to follow up and misclassification of exposure status. Cohort studies are well-suited for rare exposures and allow examination of multiple outcomes, but require large sample sizes and are time-consuming.

1. Introduction to Cohort Studies
Malimu
Kampala international University

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

3. Person
Place
Time
Cases
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10
0
200
400
600
800
1000
1200
0-4 '5-14 '15-
44
'45-
64
'64+
Age Group
Descriptive Epidemiology
Who? Where? When?

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 ?

6. Case-control
Cohort
Individuals
Intervention
Retrospective
Prospective
Descriptive
Populations
Analytical
Observational
Case-series
Cross-sectional
Correlational
Clinical trials
Epidemiological studies

7. Observational Analytic Studies
exposure outcome cohort
case-control
cross-sectional
exposure outcome
exposure outcome

8. Disease among
exposed?
Disease among
non-exposed?
Usually prospective
Cohort
study
Population
at risk
Exposed
Not Exposed
and

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

13. Exposure
occurrence
Study starts Disease
occurrence
Prospective Cohort Study
Time
+
-
+ -
ill
exp
+
-
exp
Prospective assessment
of exposure and disease
Selection of
population

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