1. Cohort Study
Dr. Win Aye Hlaing
Lecturer
Department of Epidemiology
University of Public Health, Yangon
2. Cohort Study
A cohort study is one in which a population (cohort) of persons who
are free of disease is defined at a point in time, and subsequently
classified according to the presence or absence of a determinant (e.
g. exposure to an agent). They are then observed over a period of time
to identify the subsequent appearance (incidence) of disease in those
with and without the determinant (exposure or risk factor) .
3. What is Cohort?
A cohort is a 300-600 soldiers unit in the Roman army.
4. Advantages of Cohort Study
• Proper temporal sequence of events can be observed,
thus helping to distinguish causes from associated
factors;
• New cases of disease can be identified in a specified
population and time period, which allows incidence to
be calculated;
• Several possible determinants and outcomes can be
studied simultaneously;
• Determinants and outcomes can be measured precisely
5. Disadvantages of Cohort Study
• Large population required if incidence is low;
• Long time-scale before results emerge especially if
incubation period is prolonged;
• Relatively expensive in resources;
• Losses from population during study may bias results;
• Standard methods and criteria may drift over
prolonged follow up
6. • Observational Analytic Studies
• etiologic or causal relationships
• First, association between a factor or a
characteristic and disease
• Second, appropriate inferences - possible
causal relationship
7. Design of a Cohort Study
• investigator selects a group of exposed
individuals and non-exposed individuals and
• follows up both groups to compare the
incidence of disease (or rate of death from
disease) in the two groups
8. • If a positive association
• exposed group – disease develops (incidence
in the exposed group)
• greater than the non-exposed group - disease
develops (incidence in the non-exposed
group)
11. • (a + b) exposed persons – disease develops in
a but not in b; Disease in Exposure = (a/a + b)
• (c + d) non-exposed persons - disease
develops in c but not in d;
Disease in Non-Exposure = (c/c + d)
• If exposure is associated with disease,
a/a + b ˃ c/c + d
12. • association of smoking with coronary heart
disease (CHD)
• 3,000 smokers (exposed) and
• 5,000 non-smokers (non-exposed) who are free
of heart disease at baseline
• Both groups are followed
• the incidence of CHD in both groups is compared
• CHD develops in 84 of the smokers and in 87 of
the non-smokers
• The result is an incidence of CHD
• 28.0/1,000 in the smokers and
• 17.4/1,000 in the non-smokers
13. • new (incident) cases
• whether a temporal relationship exists
between the exposure and the disease
• whether the exposure preceded the onset of
the disease
• Consider the exposure a possible cause of the
disease
14. COMPARING COHORT STUDIES
WITH RANDOMIZED TRIALS
• compare the Cohort study (observational)
with RCT (experimental)
• “exposure” in RCT is a treatment or preventive
measure
• In cohort studies, “exposure” is toxic or
carcinogenic agent
• exposed group is compared with a non-
exposed group or with a group with another
exposure
15. SELECTION OF STUDY POPULATIONS
Two basic ways
1. We can create a study population
by selecting groups for inclusion
on the basis of whether or not they were
exposed (e.g., occupationally exposed
cohorts)
16. 2. - We can select a defined population before
exposed or before exposures are identified
- select a population on the basis of some
factor not related to exposure (such a
community of residence)
17. • wait for an outcome
• often require a long follow-up period, lasting
until enough events (outcomes) have occurred
• We compare exposed and non-exposed
persons
• This comparison is Hallmark of Cohort design
18. Types of Cohort Study
• (Prospective) Cohort Study:
If the study plan calls for the cohort to be identified and
recruited “today” and followed into the future, the study
is called a prospective cohort study
• Retrospective Cohort Study:
If the study plan calls for the cohort to be selected based
on their membership in the population at a specified
point in the past, and followed from that point (in the
past) up to “today”, then the study is called a
retrospective cohort study
19. • E.g. The relationship of smoking to lung
cancer
• followed up for a long period
• to determine the outcome
• a population of elementary school students
and follow them up
• 10 years later, when they are teenagers, we
identify those who smoke and those who do
not
• follow up both groups—to see who develops
lung cancer and who does not
20. • we begin our study in 2016
• children who will become smokers within 10
years
• 10 years later, in 2026, Exposure status
(smoker or non-smoker)
• latent period from beginning smoking to
development of lung cancer is 10 years (2036)
21. • This type of study design is called a
prospective cohort study (also a concurrent
cohort or longitudinal study)
• It is concurrent because the investigator
identifies the original population at the
beginning of the study and
• accompanies the subjects concurrently
through calendar time until the point at which
the disease develops or does not develop
22. • Problem - take at least 20 years to complete
• Several problems can result
• research grant, such funding is generally
limited to a maximum of only 3 to 5 years
• subjects will outlive the investigator, or loss to
follow up due to various reasons
• the investigator may not survive to the end of
the study
• unattractive
23. • to shorten the time
• alternate approach
• we again begin our study in 2016
• but now we find that an old roster of
elementary schoolchildren from 1996
• Surveyed their smoking habits in 2006
• data resources in 2016, we can begin to
determine who has developed lung cancer
and who has not
24. • This is called a retrospective cohort (also called a
non-concurrent prospective study or historical
cohort study)
• not differ – prospective cohort design—we are
still comparing exposed and non-exposed groups
• retrospective cohort design - to use historical
data from the past
• Scope the frame of calendar time
• we are beginning the study with a pre-existing
population to reduce the duration of the study
25. • only difference between them is calendar time
• Prospective cohort design - exposure and non-
exposure - followed up for several years -
incidence is measured
• Retrospective cohort design – exposure from
past records and outcome (development or no
development of disease) at the time the study
is begun
• from objective records in the past and follow-
up and measurement of outcome
26. POTENTIAL BIASES IN COHORT STUDIES
• two major categories:
– Selection bias and
– information bias
28. Information Bias
1. Quality and Extent of information
– Historical cohort studies – from past records
2. Also knows whether that subject was
exposed
– May be biased by that knowledge
– Problem can be addressed by Masking/Blinding
3. May un-intensionally introduce biases
– Data analyses
– interpretation
29. Conclusion
• the cohort is attractive when we can minimize
attrition (losses to follow-up)
• Generally, we do not have appropriate past
records or other sources of data - to conduct a
retrospective cohort study
• Today, many of the diseases occur at very low
rates
