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The Cohort Method
The term cohort originally referred to a group of persons born the same year in the same place -- the same nation, state,
city, hospital, etc.. Cohorts were studied primarily for mortality data. Studies of births in cohorts of women born the same
year have also been used for quite some time as a way to study fertility. Today the term birth cohort is used for cohorts
of this original type.
The term cohort is used today for any group that is followed up over time. Subject are selected for inclusion in a cohort
because of some current exposure to a possible cause. They are then followed up prospectively over time to measure the incidence
of the disease under study. For this reason, cohort studies are also known as prospective studies and as incidence studies.
The Framingham Heart Study
One of the best known of all cohort studies is the Framingham Heart Study. This study was initiated in 1948 by the U.S.
Public Health Service and researchers at the Harvard School of Public Health to study a variety of possible risk factors for
heart disease. The town of Fram¬ingham in Massachusetts (population 28,000) was chosen as the site of the study because of
its relatively stable population, availability of a local community hospital, proximity to Boston and Harvard, and cooperation
in a previous community study. The study was originally planned as a 20-year followup but has now been underway almost twice
that long.
A list of all local residents was developed as a sampling frame for random selection. Families were listed together within
each precinct. Two out of every three families (6,507 persons) were invited to partic¬ipate. Of these, 4.469 persons agreed
to participate. Because this was less than the researchers" goal of 5,000 participants, a group of 740 volunteers was added
to make a total of 5,209 subjects.
All of these subjects were then given a physical examination that included measures of blood pressure and serum cholesterol.
At this stage, 82 subjects were found to have symptoms indicative of previously undiagnosed heart disease and were removed
from the sample, leaving a total of 5.127. These persons could be subdivided into subcohorts based on such characteristics
as smokers versus nonsmokers or hypertensives versus normotensives. The cohort has been offered a relatively compre¬hensive
physical examination every 2 years. This not only provides some outcome data but allows reassignment to subcohorts as conditions
change and has allowed the addition of some new measurements of interest that were not part of the original study design.
Much of our current knowledge of risk factors for heart disease either came out of this study or was confirmed by the study.
The find¬ings with regard to the role of serum cholesterol in heart disease were particularly important. In males aged 30
to 49, for instance, the risk of heart disease in those with serum cholesterol levels greater than 250 mg% is 4.1 times that
of men with cholesterol levels below 190 mg%.
The existence of this long-standing cohort, on which numerous variables have been recorded, has permitted numerous studies
that were not included in the original plan. For instance, although this cohort was created to study heart disease, the causes
of all deaths in the cohort have been recorded, allowing studies of mortality of any type and not just heart disease mortality.
For example, mortality due to all causes has been studied in relationship to alcohol consumption. Gordon and Kannel (1984)
found that there is no association between drinking and subsequent mortality in women. Among men, although increased alcohol
consumption is associated with increased mortality due to cirrhosis of the liver and cancer (especially stomach cancer), non-drinkers
suffer higher mortality than drinkers, even the heaviest drinkers. Although contrary to many peoples' expectations, these
findings are consistent with those from other cohort studies of drinking and mortality.
Coffee Consumption and Cancer Mortality
In 1966, a prospective study of 26,020 male life insurance policy-holders was initiated. A dietary frequency-of-use survey
was mailed to this population and was completed and returned by 17,818 men, who became the cohort under study. Cancer mortality
among heavy coffee users was compared to all other subjects. The hypothesized relation¬ship between heavy coffee drinking
and pancreatic cancer was not found. No positive relationships were found between coffee consump¬tion and any form of cancer
except lung cancer. Even after controlling for age, urban/rural residence, and cigarette use, the relative risk of death due
to lung cancer was 7.33 times as great for heavy coffee drinkers. This compares to a relative risk of 9.6 for cigarette smokers,
controlling for the influence of age, urban/rural residence, and coffee consumption. Persons who drink five or more cups of
coffee per day and smoke a pack or more of cigarettes per day are 40.37 times more likely to die of lung cancer.
Historical Cohorts
The major disadvantage of cohort studies is the amount of time it takes to conduct them. We often want answers now that
could only be provided by a twenty-year prospective study (RCT or cohort). Sometimes we can obtain those answers sooner by
way of a historical cohort study -- also known as a retrospective cohort study or, rather confusingly, as a prospective study
done retrospectively. The opportunity to conduct such a study is present when we can find some record that allows us to assign
some population to groups based on their exposure to a possible cause at some time in the past and to follow those people
until the present time, keeping track of the incidence of the disease under study.
Radiation and Mortality
The end of World War II thrust us into the Atomic Age with little idea of what the health consequences of radiation might
be. Although it was known that large doses of radiation can be lethal, the cumulative effects of smaller doses were unknown.
Unfortunately, to study this question through standard cohort or experimental methods would have required decades and the
answers then might come too late.
Seltser and Sartwell (1965) devised a historical cohort study to assess the possible hazards of small doses of radiation.
They studied mortality among members c F three professional organizations. The three or¬ganizations were the Radiologic Society
of North America (founded in 1915), the American College of Physicians (founded in 1915), and the American Academy of Ophthalmology
and Otolaryngology (founded in 1921). Members were studied from the time they joined their professional organization until
1958.
As radiologists, the first group had substantial exposure daily to radiation, especially during this period when modern
precautions were not taken. The second group, being composed of internists, was likely in those days to have quite a bit of
x-ray exposure but far less than the radiologists. The third group, being eye, ear, nose, and throat (ENT) specialists, made
little if any use of x-rays and thus served as a low-risk cohort.
Mortality was studied for the periods 1935-1944, 1945-1954, and 1955-1958. In all three periods, mortality rates were highest
for the radiologists and lowest for the ENT specialists. Radiologists, for instance, showed mortality due to leukemia that
was 2.5 times as great as that of the ENT specialists. Radiation exposure was clearly associated with greater mortality due
to cancer, heart disease, kidney disease, and all causes combined. The precautions taken by modern radiologists and Radiologic
technicians owe much to this study.
Recommended Reading
Cook, N. R., & Ware, J. H. (1983). Design and analysis methods for longitudinal research. Annual Review of Public Health,
4, 1-23.
Dawber, T. R., Kannel, W. B., & Lyell, L. P. (1963). An approach to longitudinal studies in a community: The Framingham
Study. Annals of the New York Academy of Sciences, 107, 593-599.
Kandel, D. B. (Ed.). (1978). Longitudinal research on drug use: Empirical findings and methodological issues. New
York: John Wiley & Sons.
Stallones, R. A. (1966). Prospective epidemiologic studies of cerebrovascular disease. In Cerebrovascular disease epidemiology:
A workshop (Public Health Monograph No. 76). Washington, DC: U.S. Government Printing Office.
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