Page 104 - Encyclopedia of Nursing Research

CoHoRT DESIGn n 71

methodology for example the quality assess- assignment of subjects to groups. Although
ment of included studies. the groups in a cohort design may not be as
In summary, Cochrane reviews pro- comparable as randomly assigned groups, C
vide consumers with readily accessible evi- archival records or data on relevant variables
dence on the effectiveness of health care can be used to compare cohorts that received
interventions. a treatment with those that did not. Because
simple comparisons between cohorts may
Josephine Hegarty suffer from a number of design problems,
Eileen Savage such as biased sample selection, intervening
historical events that may influence the out-
come variable, maturation of subjects, and
testing effects, a strong cohort design can
Cohort design account for many of these threats to the inter-
nal validity of a study.
There are two major types of cohort
A cohort design is a time-dimensional design design: the cohort design with treatment par-
to examine sequences, patterns of change or titioning and the institutional cycles design.
growth, or trends over time. A cohort is a In a cohort design with treatment partition-
group with common characteristics or expe- ing, respondents are partitioned by the extent
riences during a given time period. Cohorts of treatment (amount or length) received. In
generally refer to age groups or to groups of the institutional cycles design, one or more
respondents who follow each other through earlier cohorts are compared with the experi-
formal institutions such as universities or mental cohort on the variable(s) of interest. The
hospitals or informal institutions such as institutional cycles cohort design is strength-
a family. Populations also can be classified ened if a nonequivalent, nontreatment group
according to other time dimensions, such as is measured at the same time as the experi-
time of diagnosis, time since exposure to a mental group. A well-planned cohort design
treatment, or time since initiating a behavior. can control for the effects of age or experience
A cohort might be graduates of nurse practi- when these might confound results in a pre-
tioner programs in the years 1995, 2000, 2005, test–posttest design or when no pretest mea-
and 2010 or siblings in blended families. sures of experimental subjects are available.
Cohort designs were originally used by Cohort designs might use a combination of
epidemiologists and demographers but are cross-sectional and longitudinal data.
increasingly used in studies conducted by The term cohort studies broadly refers
nurses and other researchers in the behav- to studies of one or more cohort groups to
ioral and health sciences. examine the temporal sequencing of events
In the most restrictive sense, a cohort over time. Cohort studies may eventually
design refers to a quasi-experimental design lead to hypotheses about causality between
in which some cohorts are exposed to a variables and to experimental designs.

treatment or event and others are not. The Most cohort designs are prospective (e.g.,
purpose of a cohort design is to determine the nurses’ Health Study, in which 100,000
whether two or more groups differ on a nurses were enrolled in 1976 and have
specific outcome measure. Cohort designs been followed since), although some are
are useful for drawing causal inferences in retrospective.
quasi-experimental studies because cohort There are a number of types of cohort
groups are expected to differ only minimally studies. The panel design, in which one or
on background characteristics. Recall that more cohorts are followed over time, is espe-
a quasi-experimental design lacks random cially useful for describing phenomena.