CHAPTER 7 Study Designs: Cohort Studies<\/p>\n
LEARNING OBJECTIVES<\/p>\n
By the end of this chapter the reader will be able to:<\/p>\n
\u2022\u2022 differentiate cohort studies from other epidemiologic study designs<\/p>\n
\u2022\u2022 list the main characteristics, advantages, and disadvantages of cohort studies<\/p>\n
\u2022\u2022 describe at least three research questions that lend themselves to cohort studies<\/p>\n
\u2022\u2022 calculate and interpret a relative risk<\/p>\n
\u2022\u2022 give three examples of published studies discussed in this chapter<\/p>\n
CHAPTER OUTLINE<\/p>\n
I. Introduction<\/p>\n
II. Cohort Studies Defined<\/p>\n
III. Sampling and Cohort Formation Options<\/p>\n
IV. Temporal Differences in Cohort Designs<\/p>\n
V. Practical Considerations<\/p>\n
VI. Measures of Effect: Their Interpretation and Examples<\/p>\n
VII. Summary of Cohort Studies<\/p>\n
VIII. Conclusion<\/p>\n
IX. Study Questions and Exercises<\/p>\n
Introduction<\/p>\n
This chapter explores one of the most powerful observational epidemiologic designs\u2014the cohort study\u2014which overcomes many of the problems associated with temporality of data collection and obtaining information about exposures that are uncommon in the population. A distinguishing feature of each type of study design\u2014whether observational or experimental\u2014is the temporality of data collection with respect to exposure and disease. The term temporality refers to the timing of information gathering, that is, whether the information about cause and effect was assembled at the same time point or whether information about the cause was garnered before or after the information about the effect. When information about exposures is collected before an outcome occurs, and there is an association between exposures and outcomes, one is more confident about a possible cause-and-effect relationship. We will learn that cohort studies preserve the temporality of cause (exposure) happening before effect (disease).<\/p>\n
Cross-sectional and case-control study designs (and many types of ecologic study designs) are premised upon exposure information and disease information that are collected at the same time. In a cross-sectional study, one might administer a survey that contains questions about current or past exposures (e.g., exposure to secondhand cigarette smoke) and various outcomes (e.g., respiratory symptoms). Data for a case-control study might be collected from patient interviews and reviews of medical records. Even though exposure and outcome information are obtained simultaneously, the frame of reference for exposure assessment in case-control studies is retrospective, meaning that respondents are interviewed about exposures that occurred in the past. All in all, in case-control and cross-sectional studies, researchers obtain information about health outcomes and exposures after they have occurred.<\/p>\n
Although the strategy of collecting exposure and outcome information at the same time, and after they have occurred, is efficient for generating and testing hypotheses, the strategy does lead to almost unavoidable challenges regarding interpretation of results. In particular, cross-sectional studies present difficulties in distinguishing the causes (e.g., certain exposures) from the consequences (e.g., certain outcomes) of the disease, especially if the outcome marker is a biological or physiological parameter. Similarly, case-control studies may raise concerns that recall of past exposures differs between the cases (i.e., those study participants who have the disease or outcome of interest) and the controls (i.e., those study participants who do not). In addition, although investigators may query subjects about exposures that took place many years in the past, there has been no actual lapse of time between measurement of exposure and disease. Finally, neither cross-sectional nor case-control designs are especially well suited for exposures that are uncommon in the population. However, cohort studies overcome many of the challenges presented by temporality and uncommon exposures.<\/p>\n
Cohort Studies Defined<\/p>\n
Cohorts and Cohort Effects<\/p>\n
A cohort is defined as a population group, or subset thereof (distinguished by a common characteristic), that is followed over a period of time. The term cohort is said to originate from the Latin cohors, which is one of 10 divisions of an ancient Roman military legion. The common characteristic may be either that the group members experience an exposure associated with a specific setting (e.g., an occupational cohort or a school cohort) or that they share a nonspecific exposure associated with a general classification (e.g., a birth cohort, defined as being born in the same year or era). For example, people who belong to the same birth cohort may be exposed to similar environmental and societal changes, whereas those who belong to different birth cohorts may grow up exposed to dissimilar environmental conditions that are reflected in differences in health outcomes. The influence of membership in a particular cohort is known as a cohort effect.<\/p>\n
The term cohort analysis refers to \u201cthe tabulation and analysis of morbidity or mortality rates in relationship to the ages of a specific group of people (cohort) identified at a particular period of time and followed as they pass through different ages during part or all of their life span.\u201d1 Wade Hampton Frost helped to draw attention to the method of cohort analysis, even though he did not originate this methodology.2 Table 7\u20131 reproduces Frost\u2019s data. \u201cTo illustrate cohort analysis, Frost first arranged tuberculosis mortality rates from Massachusetts \u2026 in a table with age on one axis and year of death on the other \u2026 Arranged in this way, one could quickly see the age-specific mortality for each of the available years on one axis, and the time trend for each age group on the other. What proved to be most interesting in this instance were the rates in the cells of the table that lay on the diagonals, starting with the youngest ages and earliest years. These \u2018diagonal rates\u2019 were analogous to tuberculosis mortality rates \u2026 experienced by each cohort of persons as they simultaneously aged and passed through time.\u201d2(pp 9\u201310)<\/p>\n
Table 7\u20131 Death Rates per 100,000 from Tuberculosis, All Forms, for Massachusetts, 1880 to 1930, by Age and Sex, with Rates for Cohort of 1880 Indicated<\/p>\n","protected":false},"excerpt":{"rendered":"
CHAPTER 7 Study Designs: Cohort Studies LEARNING OBJECTIVES By the end of this chapter the reader will be able to: \u2022\u2022 differentiate cohort studies from other epidemiologic study designs \u2022\u2022 list the main characteristics, advantages, and disadvantages of cohort studies \u2022\u2022 describe at least three research questions that lend themselves to cohort studies \u2022\u2022 calculate […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[10],"class_list":["post-78532","post","type-post","status-publish","format-standard","hentry","category-research-paper-writing","tag-writing"],"_links":{"self":[{"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/posts\/78532","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/comments?post=78532"}],"version-history":[{"count":0,"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/posts\/78532\/revisions"}],"wp:attachment":[{"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/media?parent=78532"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/categories?post=78532"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/papersspot.com\/blog\/wp-json\/wp\/v2\/tags?post=78532"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}