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Cohort Studies
• All studies involve some descriptive or
analytic type of comparison of
exposure and disease status.
• Analytical study design options include:
observational or interventional (which
one is based on the role of the investigator).
• There are three basic types of
observational analytical study designs:
 Cohort studies
 Case-control studies
 Cross-sectional studies
• Specific epidemiological study designs can be
used to reveal etiologic (causal) relationships
• First, using observational analytical studies :
 Determine whether there is an association
between a factor or a characteristic and the
development of disease
• Second:
 From these associations, derive appropriate
inferences regarding a possible causal
Analytical Studies
• Control and experimental groups
• Randomized groups
 data collected without bias
• Dependent and independent factors
Cohort Studies
• Group by common characteristics
• Start with a group of subjects who lack a
positive history of the outcome of interest
yet are at risk for it (cohort).
 Think of going from cause to effect.
• The exposure of interest is determined for
each member of the cohort and the group
is followed to document incidence in the
exposed and non-exposed members.
When is a cohort
study warranted?
• When good evidence suggests an
association of a disease with a
certain exposure or exposures.
Cohort Effect
• Changes and variation in the disease or
health status of a study population as
the study group moves through time.
• “Generation effect”
Types of Cohort Studies
• Prospective (concurrent)
• Retrospective (historical)
• Restricted (restricted exposures)
Types of Cohort Studies
• Prospective – cohort characterized by
determination of exposure levels (exposed
vs. not exposed) at baseline (present) and
followed for occurrence of disease in future
 Groups move through time as they age
• Retrospective - makes use of historical
data to determine exposure level at some
baseline in the past and then determine
subsequent disease status in the present.
• Restricted - limited exposure, narrow
behavior (military, long shore men)
Prospective Studies
• Also called
 longitudinal
 concurrent
 incidence studies
• Looking into the future
• Example:
 Framingham Study of coronary heart
disease (CHD)
Design of a Cohort Experiment
• The essential characteristic in the design of
cohort studies is the comparison of
outcome in an exposed group and a
nonexposed group (or a group with a certain
characteristic and a group w/o that characteristic).
 A study population can be chosen by selecting
groups for inclusion in the study on the basis of
whether or not they were exposed
Selection of Cohort Groups
• There are two basic ways to generate cohort
 Select a cohort (defined population) BEFORE
any of its members become exposed or before
the exposures are identified.
 Select a cohort on the basis of some factor
(e.g., where they live) and take histories (e.g.,
blood tests) on the entire population to separate
into exposed and non-exposed groups.
• Regardless of which selection approach is
used, we are comparing exposed and nonexposed persons.
Design of a Cohort Experiment
Design of a Prospective
Cohort Experiment
Major problem with a prospective cohort design is that
the cohort must be followed up for a long period of time.
Framingham Study
• Designed to study the effect of multiple
factors on coronary heart disease (CHD):
elevated blood cholesterol
tobacco smoking
increased physical activity
increase in body weight
diabetes mellitus
Framingham Study Design
• Framingham, Massachusetts population
was 28,000
• Study design called for a random
sample of 6,500
• Enrollment questionnaire form targeted
age range 30-59 years
• No clinical evidence of atherosclerotic
cardiovascular disease
• Cohort re-examined every two years
• Problems: white, middle class
Hypothetical Cohort Study Approach
1. Examine people over their life times for
contraction of diseases. In meantime,
ask questions concerning diet,
lifestyle,habits, work, etc.
2. Take blood and do lab tests (as many
as possible).
3. Do same tests every year for a period
of years.
 Interventional aspect: If positive findings occur, they
refer you to a specialist. By screening, they may have
prolonged a life.
• Valid, reliable surveys
• Critical number of subjects
Garbage in,
garbage out
 the more, the better
• Randomize
 random selection
 random assignment
• Rule out bias
 For example, degree of accuracy with which
subjects have been classified with respect to
their exposure.
 For example, individuals who are sick may be
more likely to give the kind of responses that
they believe the investigator wants to hear
Data Gathering
Person - to - person
Drop off questionnaire
Mailed to people
Telephone interview
Newsletter or magazine
Potential Biases in
Cohort Studies
Information bias
Bias in estimation of the outcome
Bias from non-response
Bias from losses to follow-up
Analytic bias
Your assignment:
Describe and
differentiate between
these types of biases.
Advantages of
Prospective Cohort Studies
Captive groups
Large sample sizes
Certain diseases or risk factors targeted
Can be used to prove cause-effect
Assess magnitude of risk
Baseline of rates
Number and proportion of cases that
can be prevented
Advantages of
Prospective Studies (cont’d)
• Completeness and accuracy
• Opportunity to avoid condition being
• Quality of data is high
• Considers seasonal and other
variations over a long period
• Tracks effects of aging process
Disadvantages of
Prospective Cohort Studies
• Large study populations required
 not easy to find subjects
• Expensive
• Unpredictable variables
• Results not extrapolated to general
• Study results are limited
• Time consuming/results are delayed
• Requires rigid design and conditions
Disadvantages of
Prospective Studies (cont’d)
Subjects lost over time (dropouts)
Costs are high
Logistically demanding
Maintaining quality, validity, accuracy
and reliability can be a problem
Survivorship Studies
• Survivorship is the number of persons out of
a study population who would survive until a
certain time interval has been reached
• Shows the chance that an event (such as death
from cancer) will occur in successive intervals
of time once a diagnosis has been made
• Analysis yields a cumulative probability of
surviving the projected time period
For infectious diseases, we use case fatality rate
to assess survival
For chronic diseases, we use cohort life tables
(Cohort) Life Tables
• Charts which summarize the patterns
of survival and death in study groups
of certain types of disease (chronic)
• Insurance companies study these
charts very closely.
Survival curves and risk of death for males vs.
females based on life tables in California for 1980.
Dip at beginning of
life is due to infant
mortality rate.
As one reaches the
later years of life, the
survival curve goes
down and the risk of
death goes up.
Is the Association Causal?
To be continued…