This document describes cohort studies and their use in epidemiological research. It defines a cohort study as one that follows a group of individuals over time to examine exposure to factors and subsequent health outcomes. It notes advantages as allowing observation of temporal relationships but disadvantages as requiring large sample sizes and long follow-up periods. The document outlines the design of cohort studies, including prospective and retrospective approaches. It discusses evaluating disease incidence by comparing exposed and non-exposed groups. Finally, it covers potential biases in cohort studies from selection effects and incomplete information.

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)

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10. Disease
(+)
Disease
(-)
Exposure (+) a b a+b a/a+b
Exposure (-) c d c+d c/c+d

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

27. Selection Bias
• nonparticipation and nonresponse
• major biases Loss to follow up

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

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