Cohort Study Design In Nursing

Afza.Malik GDA

Cohort Study and Quasi Experiment Types and Problems

Cohort Study Design In Nursing

Cohort As time Dimensional Study Design,Cohort Design Refers to a Quasi Experimental Design,Types of Cohort Study Design,Problems of Cohort Study 

Cohort As time Dimensional Study Design

    A cohort design is a time-dimensional design to examine sequences, patterns of change or growth, or trends over time. A cohort is a group with common characteristics or experiences during a given time period. 

    Cohorts generally refer to age groups or to groups of respondents who follow each other through formal institutions such as universities or hospitals or informal institutions such as a family. 

    Populations also can be classified according to other time dimensions, such as time of diagnosis, time since exposure to a treatment, or time since initiating a behavior. A cohort might be graduates of nurse practitioner programs in the years 1995, 2000, 2005 or siblings in blended families. 

    Cohort designs were originally used by epidemiologists and demographers but are increasingly used in studies conducted by nurses and other researchers in the behavioral and health sciences.

Cohort Design Refers to a Quasi Experimental Design

    In the most restrictive sense, a cohort design refers to a quasi-experimental design in which some cohorts are exposed to a treatment or event and others are not. 

    The purpose of a cohort design is to determine whether two or more groups differ on a specific outcome measure. 

    Cohort designs are useful for drawing causal inferences in quasi-experimental studies because cohort groups are expected to differ only minimally on background characteristics. 

    Recall that a quasi-experimental design lacks random assignment of subjects to groups. 

    Although the groups in a cohort design may not be as comparable as randomly assigned groups, archival records or data on relevant variables can be used to compare cohorts that received a treatment with those that did not. 

    Because simple comparisons between cohorts may suffer from a number of design problems, such as biased sample selection, intervening historical events that may influence the outcome variable, maturation of subjects, and testing effects, a strong cohort design can account for many of these threats to the internal validity of a study.

 Types of Cohort Study Design 

    There are two major types of cohort design: cohort design with treatment partitioning and the institutional cycles design. 

    In a cohort design with treatment partitioning, respondents are partitioned by the extent of treatment (amount or length) received. In the institutional cycles design, one or more earlier cohorts are compared with the experimental cohort on the variable(s) of interest. 

    The institutional cycles cohort design is strengthened if a nonequivalent nontreatment group is measured at the same time as the experimental group. 

    A well-planned cohort design can control for the effects of age or experience. when these might confound results in a pre-test-post test design or when no pretest measures of experimental subjects are available. Cohort designs might utilize a combination of cross-sectional and longitudinal data.

    The term cohort studies broadly refers to studies of one or more cohort groups to examine the temporal sequencing of events over time. Cohort studies may eventually lead to hypotheses about causality between variables and to experimental designs. 

    Most cohort designs are prospective (eg, the Nurses' Health Study, in which 100,000 nurses were enrolled in 1976 and have been followed since) al- though some are retrospective.

    There are a number of types of cohort studies. The panel design, in which one or more cohorts are followed over time, is especially useful for describing phenomena. Trend studies are prospective designs used to examine trends over time. 

    In trend studies, different subsamples are drawn from a larger cohort at specified time points to look at patterns, rates, or trends over time (Polit & Hungler, 1995). 

    Panel designs with multiple cohorts are used to study change in the variable(s) of interest over time, to examine differences between cohort groups in variables, and to identify different patterns between groups. 

    In a panel study with multiple chorts , the groups can enter the study at different points in time, and the effects of aging can be differentiated from the effect of being a member of a particular cohort group (Woods & Catanzaro, 1988). 

    A prospective study is a variation of a panel design in which a cohort free of an outcome but with one or more risk factors is followed longitudinally to determine who develops the health outcome. 

    The prospective design is used to test hypotheses about risk factors for disease or other health outcomes. Some authors limit the term cohort study to designs in which exposed and non exposed subjects are studied prospectively or retrospectively from a specific point.

Problems of Cohort Study 

    A major problem with prospective studies of all types is subject attrition from death, refusal, or other forms of loss. The loss of subjects in a prospective study may lead to biased estimates about the phenomena of interests.

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