Cohort study
Encyclopedia
A cohort study or panel study is a form of longitudinal study
(a type of observational study
) used in medicine
, social science, actuarial science
, and ecology
. It is an analysis of risk factors
and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction. It is one type of clinical study design and should be compared with a cross-sectional study
. Cohort studies are largely about the life histories of segments of populations, and the individual people who constitute these segments.
A cohort
is a group of people who share a common characteristic or experience within a defined period (e.g., are born, are exposed to a drug or vaccine or pollutant, or undergo a certain medical procedure). 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.
Randomized controlled trial
s, or RCTs are a superior methodology in the hierarchy of evidence in therapy, because they limit the potential for any biases by randomly assigning one patient pool to an intervention and another patient pool to non-intervention (or placebo). This minimizes the chance that the incidence of confounding
(particularly unknown confounding) variables will differ between the two groups. However, it is important to note that RCTs may not be suitable in all cases and other methodologies would be much more suitable to investigate the study's objective
Cohort studies can either be conducted prospectively, or retrospectively from archived records.
The advantage of prospective cohort study data is that it can help determine risk factors for contracting a new disease because it is a 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 follow-up time to generate useful data. Nevertheless, the results that are obtained from long-term cohort studies are of substantially superior quality to retrospective/cross-sectional studies, and cohort studies are considered the gold standard in observational epidemiology. Moreover, cohort studies are informative for efficiently studying a wide-range of exposure-disease associations.
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.
(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 trial
s, 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 minimizes 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.
Two examples of cohort studies that have been going on for more than 50 years are the Framingham Heart Study
and the National Child Development Study
(NCDS), the most widely-researched of the British birth cohort studies
.
Key findings of NCDS and a detailed profile of the study appear in the International Journal of Epidemiology.
International Journal of Epidemiology comparison of two Cohorts, Millennium Cohort Study (United States) and the The King’s Cohort (United Kingdom)
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.
Other famous examples are the Grant Study
tracking a number of Harvard graduates from ca. 1950.77, and the Whitehall Study
tracking 10,308 British civil servants.
.
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.
(since 1991), the Household, Income and Labour Dynamics in Australia Survey
(since 2001) and the European Community Household Panel (1994–2001).
Longitudinal study
A longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the...
(a type of observational study
Observational study
In epidemiology and statistics, an observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator...
) used in medicine
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, social science, actuarial science
Actuarial science
Actuarial science is the discipline that applies mathematical and statistical methods to assess risk in the insurance and finance industries. Actuaries are professionals who are qualified in this field through education and experience...
, and ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
. It is an analysis of risk factors
Risk factors
A risk factor is a concept in finance theory such as the CAPM, APT and other theories that use pricing kernels. In these models, the rate of return of an asset is a random variable whose realization in any time period is a linear combination of other random variables plus a disturbance term or...
and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction. It is one type of clinical study design and should be compared with a cross-sectional study
Cross-sectional study
Cross-sectional studies form a class of research methods that involve observation of all of a population, or a representative subset, at one specific point in time...
. Cohort studies are largely about the life histories of segments of populations, and the individual people who constitute these segments.
A cohort
Cohort (statistics)
In statistics and demography, a cohort is a group of subjects who have shared a particular time together during a particular time span . Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e...
is a group of people who share a common characteristic or experience within a defined period (e.g., are born, are exposed to a drug or vaccine or pollutant, or undergo a certain medical procedure). 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.
Randomized controlled trial
Randomized controlled trial
A randomized controlled trial is a type of scientific experiment - a form of clinical trial - most commonly used in testing the safety and efficacy or effectiveness of healthcare services or health technologies A randomized controlled trial (RCT) is a type of scientific experiment - a form of...
s, or RCTs are a superior methodology in the hierarchy of evidence in therapy, because they limit the potential for any biases by randomly assigning one patient pool to an intervention and another patient pool to non-intervention (or placebo). This minimizes the chance that the incidence of confounding
Confounding
In statistics, a confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable...
(particularly unknown confounding) variables will differ between the two groups. However, it is important to note that RCTs may not be suitable in all cases and other methodologies would be much more suitable to investigate the study's objective
Cohort studies can either be conducted prospectively, or retrospectively from archived records.
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 effect; 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 follow a group of people who do not have the disease for a period of time and see who develops the disease (new incidence). The cohort cannot therefore be defined as a group of people who already have the disease. Prospective (longitudinal) cohort studies between exposure and disease strongly aid in studying causal associations, though distinguishing true causality usually requires further corroboration from further experimental trials.The advantage of prospective cohort study data is that it can help determine risk factors for contracting a new disease because it is a 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
Attrition (medicine, epidemiology)
In science, attrition are ratios regarding the loss of participants during an experiment. Attrition rates are values that indicate participant drop out. Higher attrition rates are found in longitudinal studies....
and take a long follow-up time to generate useful data. Nevertheless, the results that are obtained from long-term cohort studies are of substantially superior quality to retrospective/cross-sectional studies, and cohort studies are considered the gold standard in observational epidemiology. Moreover, cohort studies are informative for efficiently studying a wide-range of exposure-disease associations.
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.
Examples
An example of an epidemiological question that can be answered by the use of a cohort study is: does exposure to X (say, smoking) associate 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 variableIndependent variable
The terms "dependent variable" and "independent variable" are used in similar but subtly different ways in mathematics and statistics as part of the standard terminology in those subjects...
(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
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
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
Case-control
A case-control study is a type of study design in epidemiology. Case-control studies are used to identify factors that may contribute to a medical condition by comparing subjects who have that condition with patients who do not have the condition but are otherwise similar .Case-control studies are...
study.
Shorter term studies are commonly used in medical research as a form of clinical trial
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...
, 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 trial
Randomized controlled trial
A randomized controlled trial is a type of scientific experiment - a form of clinical trial - most commonly used in testing the safety and efficacy or effectiveness of healthcare services or health technologies A randomized controlled trial (RCT) is a type of scientific experiment - a form of...
s, 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 minimizes 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.
Two examples of cohort studies that have been going on for more than 50 years are the Framingham Heart Study
Framingham Heart Study
The Framingham Heart Study is a long-term, ongoing cardiovascular study on residents of the town of Framingham, Massachusetts. The study began in 1948 with 5,209 adult subjects from Framingham, and is now on its third generation of participants...
and the National Child Development Study
National Child Development Study
The National Child Development Study is a continuing, multi-disciplinary longitudinal study which follows the lives of about 17,000 people born in Great Britain in a certain week in 1958.-History:...
(NCDS), the most widely-researched of the British birth cohort studies
British birth cohort studies
Birth cohort studies in Britain include four long-term medical and social studies, carried out over the lives of a group of participants, from birth. Two of these studies have continued for over 50 years.-Principal cohort studies:...
.
Key findings of NCDS and a detailed profile of the study appear in the International Journal of Epidemiology.
International Journal of Epidemiology comparison of two Cohorts, Millennium Cohort Study (United States) and the The King’s Cohort (United Kingdom)
The largest cohort study in women is the Nurses' Health Study
Nurses' Health Study
The Nurses Health Study, established in 1976 by Dr. Frank Speizer, and the Nurses' Health Study II, established in 1989 by Dr. Walter Willett, are the most definitive long-term epidemiological studies conducted to date on older women's health. The study has followed 121,700 female registered...
. 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
Birth to Twenty
Birth to Twenty is Africa's largest and longest running study of child and adolescent health and development, and one of the few large-scale longitudinal studies in the world. The study is a birth cohort study, and in 1990 began to track the development of 3,273 newborn infants...
Study which began in 1990 and tracks a cohort of over 3,000 children born in the weeks following Nelson Mandela
Nelson Mandela
Nelson Rolihlahla Mandela served as President of South Africa from 1994 to 1999, and was the first South African president to be elected in a fully representative democratic election. Before his presidency, Mandela was an anti-apartheid activist, and the leader of Umkhonto we Sizwe, the armed wing...
's release from prison.
Other famous examples are the Grant Study
Grant Study
The Grant Study is a 68-year longitudinal study of two socially different cohorts: 237 physically and mentally healthy Harvard college sophomores from the classes of 1939-1944, and a second cohort of 332 disadvantaged non-delinquent inner-city youths who grew up in Boston neighborhoods between 1940...
tracking a number of Harvard graduates from ca. 1950.77, and the Whitehall Study
Whitehall Study
The original Whitehall Study investigated social determinants of health, specifically the cardiorespiratory disease prevalence and mortality rates among British male civil servants between the ages of 20 and 64. The initial prospective cohort study, the Whitehall I Study, examined over 18,000...
tracking 10,308 British civil servants.
Retrospective cohort
A "prospective cohort" defines the groups before the study is done, while a "retrospective cohort" defines the grouping after the data is collected. Examples of a retrospective cohort are Long-Term Mortality after Gastric Bypass Surgery. and The Lothian Birth Cohort StudiesThe Lothian Birth Cohort Studies
The Lothian birth-cohort studies comprise three Cohort studies, combining research into psychology, epidemiology and geriatrics.-Lothian Birth Cohort Study 1921:...
.
Nested case-control study
An example of a nested case-control studyNested case-control study
A nested case control study is a variation of a case-cohort study in which only a subset of controls from the cohort are compared to the incident cases. In a case-cohort study, all incident cases in the cohort are compared to a random subset of participants who do not develop the disease of interest...
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
Framingham Heart Study
The Framingham Heart Study is a long-term, ongoing cardiovascular study on residents of the town of Framingham, Massachusetts. The study began in 1948 with 5,209 adult subjects from Framingham, and is now on its third generation of participants...
cohort.
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 SurveyBritish Household Panel Survey
The British Household Panel Survey , carried out at the Institute for Social and Economic Research of the University of Essex, is an instrument for social and economic research. A sample of British households was drawn and first interviewed in 1991. The members of these original households have...
(since 1991), the Household, Income and Labour Dynamics in Australia Survey
Household, Income and Labour Dynamics in Australia Survey
The Household, Income and Labour Dynamics in Australia survey is an Australian household-based panel study which began in 2001. It has been used for examining issues such as the incidence of persistent poverty; assets and income in the transition to retirement; the correlates and impact of...
(since 2001) and the European Community Household Panel (1994–2001).
External links
- Prospective cohorts
- Retrospective cohorts
- Study Design Tutorial Cornell University College of Veterinary Medicine
- Birth cohort study timelines (ESDS Longitudinal)
- Centre for Longitudinal Studies