Cohort study

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A cohort study or panel study is a form of longitudinal study
used in medicine and social science. It is one
type of study design and should be
compared with a cross-sectional
study.

A cohort is a group of
people who share a common characteristic or experience within a defined period
(e.g., are born, leave school, lose their job, are exposed to a drug or a
vaccine, etc.). Thus a group of people who were born on a day or in a particular
period, say 1948, form a birth cohort. The comparison group may be the general
population from which the cohort is drawn, or it may be another cohort of
persons thought to have had little or no exposure to the substance under
investigation, but otherwise similar. Alternatively, subgroups within the cohort
may be compared with each other.

Contents

1 Application

2 Examples

3 Variations

3.1 Retrospective
cohort

3.2 Nested case-control
study

3.3 Household panel
survey

4 See also

5 References

6 External links

[edit]
Application

In medicine, a cohort study is often undertaken to obtain evidence to try to
refute the existence of a suspected association between cause and disease;
failure to refute a hypothesis strengthens confidence in it. Crucially, the
cohort is identified before the appearance of the disease under investigation.
The study groups, so defined, are observed over a period of time to determine
the frequency of new incidence of the studied disease among them. The cohort
cannot therefore be defined as a group of people who already have the disease.
Distinguishing causality from mere correlation cannot usually be done with
results of a cohort study alone.

The advantage of cohort study data is the longitudinal observation of the
individual through time, and the collection of data at regular intervals, so
recall error is reduced. However, cohort studies are expensive to conduct, are
sensitive to attrition
and take a long time to generate useful data.

Some cohort studies track groups of children from their birth, and record a
wide range of information (exposures) about them. The value of a cohort study
depends on the researchers' capacity to stay in touch with all members of the
cohort. Some of these studies have continued for decades.

[edit]
Examples

An example of an epidemiologic question that can be answered by the use of a
cohort study is: does exposure to X (say, smoking) correlate with outcome Y
(say, lung cancer)? Such a study would recruit a group of smokers and a group of
non-smokers (the unexposed group) and follow them for a set period of time and
note differences in the incidence of lung cancer between the groups at the end
of this time. The groups are matched in terms of many other variables
such as economic status and other health status so that the variable being
assessed, the independent
variable (in this case, smoking) can be isolated as the cause of the dependent variable
(in this case, lung cancer).

In this example, a statistically
significant increase in the incidence of lung cancer in the smoking group as
compared to the non-smoking group is evidence in favor of the hypothesis.
However, rare outcomes, such as lung cancer, are generally not studied with the
use of a cohort study, but are rather studied with the use of a case-control study.

Shorter term studies are commonly used in medical research as a form of clinical trial, or means
to test a particular hypothesis of clinical importance. Such studies typically
follow two groups of patients for a period of time and compare an endpoint or
outcome measure between the two groups.

Randomized
controlled trials, or RCTs are a superior methodology in the hierarchy of
evidence, because they limit the potential for bias by randomly assigning one
patient pool to an intervention and another patient pool to non-intervention (or
placebo). This minimises the chance that the incidence of confounding variables
will differ between the two groups.

Nevertheless, it is sometimes not practical or ethical to perform RCTs to
answer a clinical question. To take our example, if we already had reasonable
evidence that smoking causes lung cancer then persuading a pool of non-smokers
to take up smoking in order to test this hypothesis would generally be
considered quite unethical.

An example of a cohort study that has been going on for more than 50 years is
the Framingham Heart
Study.

The largest cohort study in women is the Nurses' Health
Study. Started in 1976, it is tracking over 120,000 nurses and has been
analyzed for many different conditions and outcomes.

The largest cohort study in Africa is the Birth to Twenty Study
which began in 1990 and tracks a cohort of over 3,000 children born in the weeks
following Nelson Mandela's release
from prison.

[edit]
Variations

[edit]
Retrospective cohort

A "prospective cohort"
defines the groups before the study is done, while a "retrospective
cohort" does the grouping after the data is collected. Whereas prospective cohorts
should be summarized with the relative risk, retrospective
cohorts should be summarized with the odds ratio. An example of a
retrospective
cohort is Long-Term Mortality after Gastric Bypass Surgery.[1]

[edit]
Nested case-control study

Main article: Nested
case-control study

An example of a nested
case-control study is Inflammatory markers and the risk of coronary heart
disease in men and women, which was a case control analyses extracted from
the Framingham Heart
Study cohort.[2]

[edit]
Household panel survey

Household panel surveys are an important sub-type of cohort study. These draw
representative samples of households and survey them, following all individuals
through time on a usually annual basis. Examples include the US Panel
Study on Income Dynamics (since 1968), the German
Socio-Economic Panel (since 1984), the British
Household Panel Survey (since 1991), the Household,
Income and Labour Dynamics in Australia Survey (since 2001) and the European
Community Household Panel (1994-2001).

[edit]
See also

Panel analysis

[edit]
References

^
Adams TD, Gress RE, Smith SC, et al (2007).
"Long-term mortality after gastric bypass surgery". N. Engl. J. Med.
357 (8): 753–61. doi:10.1056/NEJMoa066603. PMID
17715409.

^
Pai JK, Pischon T, Ma J, et al (2004).
"Inflammatory markers and the risk of coronary heart disease in men and
women". N. Engl. J. Med. 351 (25): 2599–610. doi:10.1056/NEJMoa040967. PMID
15602020.

[edit]
External links

Prospective cohorts

Retrospective cohorts

Born in Bradford - an
example of a 2006 UK-based birth cohort study

v • d • e

Biomedical
research: Study designs /
Design of
experiments

Overview

Clinical
trial · Clinical
trial protocol · Clinical
trial management ·
Academic
clinical trials · Study
design

Controlled
study
(EBM
I to II-1; A to B)

Randomized
controlled trial (Blind
experiment, Open-label
trial)

Observational
study
(EBM
II-2 to II-3; B to C)

Cross-sectional
study vs. Longitudinal
study

Cohort study (Retrospective
cohort study, Prospective
cohort study)

Case-control
study (Nested
case-control study)
Case series · Case study/Case
report

Epidemiology/
methods

Occurrence:
Incidence
(Cumulative
incidence) · Prevalence (Point
prevalence, Period
prevalence)

Association:
Relative
risk · Odds ratio · Hazard
ratio · Attributable
risk
other: Virulence · Infectivity · Mortality
rate · Morbidity · Case
fatality · Sensitivity
and specificity

Trial/test types

In
vitro/In vivo · Animal
testing · First-in-man
study · Multicenter
trial · Seeding
trial · Vaccine
trial

Analysis
of clinical trials

Risk-benefit
analysis

Interpretation of results

Selection
bias · Correlation
does not imply causation · Null
result

Category •
Glossary •
List
of topics

Retrieved from "http://en.wikipedia.org/wiki/Cohort_study"

Categories: Epidemiology
| Experimental
design | Evaluation
methods | Research
methods | Statistical
data types

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This page was last modified on 10 December 2008, at 00:34.

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