Often called “the cornerstone” of public health, epidemiology is the study of the distribution and determinants of diseases, health conditions, or events among populations and the application of that study to control health problems. By applying the concepts learned in this course to current public health problems and issues, students will understand the practice of epidemiology as it relates to real life and makes for a better appreciation of public health programs and policies. This course explores public health issues like cardiovascular and infectious diseases – both locally and globally – through the lens of epidemiology.
Cohort Study Design
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Do curso por The University of North Carolina at Chapel Hill
Epidemiologia: A Ciência Básica da Saúde Pública
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The University of North Carolina at Chapel Hill
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Study Designs
This module introduces the following study designs: experimental, cohort, case control, cross-sectional, and ecologic.
Conheça os instrutores
Dr. Karin YeattsClinical Associate Professor
Department of Epidemiology, UNC Gillings School of Global Public Health
Dr. Lorraine AlexanderClinical Associate Professor, Director of Distance Learning (North Carolina Institute for Public Health)
Department of Epidemiology, UNC Gillings School of Global Public Health
[MUSIC]
Cohort studies are a type of observational study.
We would like you to learn this study design particularly well.
The cohort study provides the foundation for
understanding other types of observational study designs.
With the cohort study design, researchers follow
an at-risk study population over time, and
evaluate exposures over time, and determine the
subsequent risk or rate of disease or
health outcome.
In this segment, we will cover the following learning objectives.
First, to explain the definition of a cohort.
Next, to recognize which measures of disease or health
outcome occurrence are used in a cohort study design.
You will also learn how to distinguish between open
and closed cohorts, as well as prospective and retrospective cohorts.
And lastly, you'll be able to list the advantages and disadvantages
of cohort studies.
Historically, the word cohort was used to describe
a subunit of a Roman legion of soldiers.
I mention this historic reference as it might, may help you
remember the image of a group of people marching through time.
A cohort is typically defined as a
group of persons sharing a common characteristic.
Epidemiologists may define their cohort by any number of shared factors.
Some examples of a common characteristic are geographic location,
occupation, socioeconomic status, age, gender, or race or ethnicity.
A famous example of a cohort study was initiated in
the early 1950s by Richard Doll and Sir Austin Bradford Hill.
Hill and Bradford began a 20-year
study following a cohort of British physicians.
The common characteristic defining this cohort was that they were
male doctors whose names were in the 1951 British Medical Register.
Thus, the characteristics used to assemble the
cohort were occupation, physicians, and geographic location, Britain.
At the outset of the investigation, the relevant demographic
exposure or other factors were determined for each subject.
The main baseline exposure of interest in this study was smoking.
Specifically, tobacco cigarette smoking.
Smoking was determined by questionnaires at baseline.
The study subjects were asked if they were
current smokers, past smokers, or had never smoked before.
The outcome of interest was mortality or death.
The mortality data was obtained from the Register's General of the
United Kingdom and complimented with
records from the British Medical Association.
In the United States, you may have heard
of cohorts such as the Women's Health Initiative.
This cohort was comprised of 93,676 post menopausal women
between the ages of 50 and 79, and followed for approximately eight years.
Other cohorts include the National Children's Study, a
cohort of pregnant women in the United States.
Or the Framingham Heart Study.
What do all these cohorts have in common?
They were assembled with the common characteristics,
such as age and gender in mind.
These cohorts then had key exposure characteristics assessed at baseline.
And then the people participating were followed over time.
The basic design of a cohort's study
ideally begins with a well designated source population.
Either study does a whole or a randomly selected sample.
For example, all physicians included in the 1951 British Medical Register.
Or, a random sample of male physicians whose
names were included in the 1951 British Medical Register.
This population,
or random sample, is then assessed, and subjects are removed who already have
the outcome or disease of interest, such as lung cancer or cardiovascular disease.
Or who don't meet whatever predesignated
inclusion criteria that investigators have decided upon.
The goal here is that the eligible study
population both accurately and efficiently represent the source population.
For example, in a cohort study
of prostate cancer, you would not likely want to include women.
One criteria of our source population for this example would therefore be gender.
So let's go over the cohort study basics.
Cohort studies track participants over time.
The subjects in a cohort study are selected to be
free of the outcome of interest at the study onset.
So it is clear that the exposure precedes the outcome.
The exposure of interest is measured in all subjects at baseline,
and/or at regular time points dur, during the course of the study.
Once the cohort is assembled and baseline exposures
are measured, then study subjects are followed over time.
The occurrence of this specific disease or
how the outcome of interest is followed closely.
New outcome events, such as incident cases of
disease, death or a health status change accounted for
all measures of the cohort throughout the follow-up period.
New cases of the outcome are used to calculate whatever measures of
incidents are relevant to the study, usually a risk or a rate.
In the cohort of British physicians we mentioned earlier,
the main exposure of interest in the study was smoking.
It was determined by questionnaire at baseline.
The study subjects were asked if they were
current smokers, past smokers, or had never smoked before.
The outcome of interest was mortality or death.
And investigator may select a cohort specifically
to study certain uncommon or rare exposures.
In some cohort studies, population groups with known exposures to
a suspected hazardous substance or environment are first identified and
recruited for study.
And then another population or group without that exposure is identified.
And the risk or rate of that outcome over time is compared in the two groups.
For this reason, cohort studies can be
particularly useful for studying uncommon or rare exposures,
because usually it's possible to identify and assemble
groups of persons who have that uncommon exposure.
Other studies may create categories, such as amalgam
of risk factors for the disease or outcome under study.
The famous Framingham Cardiovascular Cohort Study provided much of the evidence
for what is known today regarding the risks of heart disease.
The study subjects were initially categorized according to
suspected risk factors, creating risk groups for comparison.
These risk groups were then followed for 20 to 30 years.
The development
of cardiovascular disease among the various risk
groups was then compared with statistical analysis.
A cohort study of US Air Force veterans from the Vietnam War was set up to
examine the effects of exposure to Agent Orange,
a defoliant dropped by planes during the campaign.
This group of veterans were compared to air force pilots active
at the time with no involvement in the Agent Orange campaign.
Attempting to conduct this study in the general population would have
not been possible as exposure to Agent Orange is too rare.
A common measure of health outcome occurrence in
a cohort study is a risk or a rate.
Since cohort studies are chosen to be free
of the outcome of interest at the outset, only
new health outcome events, such as diseases, behavior changes,
injuries, or even improvement in health status are considered.
Note that some cohort studies of diseased persons have been conducted.
For example, persons with arthritis.
The outcome of these studies is not the
development of the disease, but rather the consequences
of having the disease, such as development of
heart disease in persons with different types of arthritis.
Or mortality differences between people with different types of arthritis.
Or quality of life.
In these cases, one could consider the disease type the, the exposure.
For risk, the total number of disease-free persons in the cohort is the denominator.
For a rate, we only count person time at risk in the denominator.
This is estimated by calculating the amount of time
each person contributes to the study free of disease.
When a subject develops the outcome of
interest or disease, he or she is no longer contributing to person time.
Similarly, those subjects lost to follow-up do not contribute person time
as the investigator is unable to determine their health outcome status.
We will now discuss the type of study population followed in a cohort study.
Cohort study populations can be open or closed.
In an open cohort,
individuals are allowed to join the study at any point
in time, from the beginning to the end within limitations.
In a closed cohort, the entire cohort is formed at the
beginning of the study and the cohort is closed to new participants.
An open study population collects person-time.
An open study population is also less prone
to problems with sample size, because study subjects
can contribute person time, even if they are in the study only for a short time.
There are two types of cohort studies, retrospective and prospective.
And they are classified according to their temporal sequence.
Retrospective and prospective refer to the time the
investigator initiates the study and starts collecting data.
Both designs assemble cohorts on the basis of exposure first.
In the retrospective study, the cohort is formed in the past.
The prospective study starts now and goes into the future.
In a prospective study, the investigators obtain baseline
exposure data in real time, and then follow the
cohort members during the time after baseline exposure to
measure the occurrence of the health outcome or disease.
The retrospective or historical cohort study is often used
to evaluate occupational exposure, such as cancer and other
chronic diseases in workers exposed to potentially hazardous substances.
An example might be deaths from lung cancer among asbestos exposed workers.
Retrospective cohort studies are possible when historical records exist to identify
the important baseline characteristics of study subjects from prior years, i.e.,
the list of workers employed
at an asbestos mine between 1930 and 1940.
The mortality experience of these workers can be traced
through vital statistics and medical records from the baseline
years to the present, and then compared with the
similar non asbestos mining cohort, or with the general population.
Using the British Doctors study example again, we have a prospective cohort study.
Prospective
or concurrent cohort studies access the baseline exposure in
real time, and then cohorts are followed into the future.
In this study, baseline measurements were made in real time of physicians'
smoking habits as of 1951 when they were, received their initial questionnaire.
Physicians were followed over time by a mailed
questionnaire sent out every five or ten years.
And their mortality was tracked
by extensive records kept on physicians in the United Kingdom.
Does smoking exposure precede mortality?
Because exposure status was determined at
baseline among living members of the British
Medical Register in the study, we
are certain that exposure preceded the outcome.
Now let's talk about the advantages of cohort studies.
We will discuss measures of association, such as risk ratios and
risk differences in next week's module.
So let's summarize by considering the
advantages and disadvantages of cohort studies.
A big advantage of cohort studies is that
they allow direct estimations of risks or rates.
Investigators may specifically seek out individuals
for study with an exposure that is
not typical among the general population, as you remember, our Agent Orange example.
The ability to assess the effects of rare
exposures is an advantage of the cohort study.
Cohort studies can also be useful for assessing multiple outcomes.
The various causes of mortality assessed in the British Physician studies
illustrates the ability to assess multiple outcomes of a single exposure.
This study examined all reported causes of mortality as
per the current International List of Causes of Death.
The researchers summarized their conclusions by
identifying excess mortality among smokers by cause.
A big disadvantage of cohort studies is that they are expensive.
They are also time consuming.
Our ability to detect relatively small differences
in risks and rates between exposed and
unexposed groups is primarily influenced by the
number of out, health outcomes in each group,
rather than the number of persons in each exposed group.
Thus, if there are relatively few persons in an
exposure category, we may need a very long period
of follow-up to observe sufficient numbers of rarer outcomes
in order to detect differences across levels of exposure.
This accounts for the considerable cost and
time needed to properly conduct a cohort study.
If outcomes are very
rare, then the size of the cohort groups may be
too large to effectively detect a difference between study groups.
Examples of rare outcomes include certain cancers,
such as acute leukemia or a kidney cancer.
Losses to follow-up occur when we can not determine the outcome
for some measures of the cohort during the entire course of follow-up.
If losses are greater in the exposed versus the unexposed
group or vice versa, we may obtain a biased
estimate of the risk ratio or ra, rate ratio.
This concludes our segment on cohort study designs.
Of all the study designs, we recommend that you learn this one particularly
well because it's the basis for learning about all the other study designs.
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