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Cohort Studies ERIC NOTEBOOK SERIES Second Edition Cohort Studies A cohort study is a type of categories once baseline Second Edition Authors: epidemiological study in which a measurements of a defined group of people with a common population are made. For example, Lorraine K. Alexander, DrPH characteristic is followed over time the Framingham Cardiovascular to find how many reach a certain Disease Study (CVD) used baseline Brettania Lopes, MPH health outcome of interest (disease, measurements to divide the condition, event, death, or a change population into categories of CVD risk Kristen Ricchetti-Masterson, MSPH in health status or behavior). A factors. cohort is defined as a group of For instance, an investigator wants to Karin B. Yeatts, PhD, MS persons, usually 100 or more in size, study whether exposure to military who share a common characteristic, aircraft engine noise is a risk factor for e.g. smokers, workers in a lead hearing loss. The cohort this smelter, people born in the same investigator would want to establish year, or all enrollees of a specific should be composed of two groups of health insurance plan Cohort military personnel: one exposed to studies compare an exposed group engine aircraft noise (the group under of individuals to an unexposed (or study) and the other unexposed to less exposed) group of individuals to engine aircraft noise (a comparison determine if the outcome of interest group). The unexposed group should is associated with exposure. There be representative of the exposed are two types of cohort studies: prospective and retrospective (or group on all factors except exposure. historical) cohorts. Prospective The cohort at baseline studies follow a cohort into the future for a health outcome, while After the cohort of study subjects is retrospective studies trace the established, their individual exposures cohort back in time for exposure of interest are identified at baseline information after the outcome has (through interviews, questionnaires, occurred. Both types of cohort bioassays, medical records, etc.). studies are also referred to as Subjects with the outcome of interest at baseline are excluded. Therefore, longitudinal or follow-up studies. all members of the cohort are at risk Establishing the cohort of developing the outcome at the The investigator controls the beginning of observation. selection of the cohort. The Following the last example, anyone in investigator may choose a cohort the cohort of military personnel with a based on age, location, exposure to specified hearing loss at baseline a certain working environment, or would be excluded from the cohort some other common characteristic. and would not be followed. Cohorts may be selected on the basis of exposures known at Following the cohort baseline, e.g. smokers vs. The cohort is then followed over time nonsmokers. Alternatively, cohorts for new occurrences of the outcome of may be divided into exposure ERIC at the UNC CH Department of Epidemiology Medical Center ERIC NOTEBOOK PA G E 2 interest, in the above example, hearing loss. In a Evaluation of the results prospective, or concurrent, cohort study baseline exposure During the follow-up period the investigator counts the is assessed at the beginning of the study and the cohort is number of subjects who develop the outcome of interest. followed into the future. In a retrospective, or historical This count is the numerator for a calculation of risk, also cohort study, baseline exposure is assessed at some point referred to as cumulative incidence or incidence in the past through historical records, e.g. health records proportion. The number of persons at risk at baseline is for a cohort of factory workers may provide exposure and the denominator. outcome information up to the present. Cohort Baseline Follow-up Risk (also called Cumulative Incidence or Incidence Prospective Assessed at begin- Followed into the Proportion) = new occurrences of the outcome / pop- ulation-at-risk at baseline ning of study future for out- come Two risks can be compared to provide a risk ratio. The Retrospec- Assessed at some Outcome has reference group is a comparable unexposed cohort. The tive point in the past already occurred index group is the exposed cohort. The risk ratio is via historical rec- and is assessed computed by dividing the risk in the exposed group by the risk in the unexposed group. The risk ratio gives a relative ords via historical rec- measure of the increase or decrease in incidence ords between the exposed and unexposed groups. Cohorts are followed over time to the end of follow-up. Occurrence of the outcome of interest may be determined via interviews with members of the cohort and/or family members, or by viewing health and/or work records to As with risk, an incidence rate measure (IR) is conclude the study. calculated with occurrences of the outcome as the numerator. An incidence rate is also called incidence density. in an IR calculation the denominator is person-time (days, months, years) at risk during follow-up Person-time is measured by summing total time each member of the cohort was free of outcome of interest and thus contributed to at-risk during the follow-up period. The IR the rapidity of occurrence of new health outcomes in Two IRs may also be compared to find the increase or decrease in the rate of health outcome between the exposed and unexposed groups. relative measure is called the incidence rate The basic design of a cohort study from beginning of the study to end of follow-up. E = exposed, Ē = not exposed, D = diseased and D = not diseased. Incidence measures between exposed and cohorts can also be subtracted from one another to the difference between the two ERIC at the UNC CH Department of Epidemiology Medical Center ERIC NOTEBOOK PA G E 3 measures. This is referred to as a risk observational study design used to help establish difference or a rate cause effect Disadvantages of cohort studies Cohort studies require large sample sizes, especially when the is rare, defined as less than 1 event per 1000 years (e.g., all specific Exposure may be a risk factor or a factor in cancers). Therefore, studies tend to be the development of the outcome of When expensive and When there are losses exposure is preventive, the risk ratio or will to follow-up (individuals leave the cohort before the be than end of follow-up) biases occur. Thus, individuals who leave the prematurely may have a different Advantages of a cohort study baseline risk than members who remain in the cohort throughout the length of follow-up. A cohort study directly measure the risk Therefore, the study may not generalizable to the and of a health outcome occurrence over time original target population, but those who Cohort studies are an efficient means of studying remained under investigation the length of exposures (e.g. gasoline fumes, as discussed in the the study. Also, any differences in quality of paragraph), in contrast to case-control studies, measurement of exposure or disease exposed tend to be better for rare outcomes. Cohort and non-exposed cohorts may information studies allow the investigator to assess multiple bias and thereby distort the outcomes of single A cohort study would be the most efficient Practice Questions of studying the effects of long-term exposure Answers are located at end of this notebook. gasoline fumes. The cohort would consist individuals who are exposed daily to Researchers are conducting a prospective cohort study of fumes (auto mechanics, gas station attendants, the association between being an office worker who uses crewman on tankers, etc.). By studying this a computer daily and carpal tunnel syndrome (a hand/ arm nerve condition). of individuals, the investigator can better 1) Researchers choose a group of office workers from the direct effects of long-term, regular one company to be their “exposed” group. Which of inhalation. Also, by conducting a cohort the following “unexposed or less exposed” study, investigator could determine if gasoline comparison groups would be the best choice? causes many different health outcomes different types of cancer and respiratory a) A group of high school students, who use computers daily Additional advantages of cohort studies b) Another group of office workers at a different company, who do not use computers daily studies establish temporal relationships exposure and outcome. Exposure clearly c) Another group of office workers at the same company, precedes outcome because the population under who do not use computers daily study at is free of the outcome of interest. Cohort studies avoid recall bias (as the exposure d) A group of professional golfers, who use their hands/ is determined the outcome, one's health arms intensively for their sport outcome or disease state won't affect accurately one recalls exposure levels), as well survival bias (duration of disease influencing 2) Who should be excluded from the cohort (not included measurements). Therefore, cohort studies are the in the study)? ERIC at the UNC CH Department of Epidemiology Medical Center ERIC NOTEBOOK PA G E 4 3) A total of 300 exposed and 300 unexposed participants are enrolled and followed for 10 years. A total of 25 Acknowledgement exposed and 17 unexposed had the outcome of interest over the follow-up period. The authors of the Second Edition of the ERIC Notebook would like to a) What is the risk of developing carpal tunnel syndrome acknowledge the authors of the ERIC among the exposed? Notebook, First Edition: Michel Ibrahim, MD, PhD, Lorraine Alexander, b) What is the risk of developing carpal tunnel syndrome DrPH, Carl Shy, MD, DrPH and Sherry among the unexposed? Farr, GRA, Department of Epidemiology at the University of c) What is the risk ratio for developing carpal tunnel North Carolina at Chapel Hill.
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