Page 545 - Encyclopedia of Nursing Research

512 n TIME SERIES ANALYSIS

They are appropriate to cyclical patterns as response patterns of individuals, families,
well as periodic or systematic variance across communities, health care systems, or politi-
T time. Outcomes of nursing care are generally cal institutions.
quantified by measures of response changes The characteristic feature of time series
across specified periods of time: improve- analysis is that the phenomenon to be stud-
ment or declines in health status, increase or ied has a distinctive temporal component—
decrease in strength or endurance are a few the behavioral state will vary predictably
examples. Although these changes are often with the passage of time. Obviously, the pas-
treated as simple, linear processes, the rate sage of time cannot be manipulated, thus,
and degree of linear variation in outcome differences in patterns of change are not a
variables are often confounded by related direct function of time. Instead, time is the
or underlying predicable patterns of fluctu- necessary temporal frame or marker in any
ation, Thus, whereas time series statistical time series analysis study. Although not
models are an appropriate and powerful conceptually an independent variable, time
methodology for analysis of intraindividual assumes that role in univariate time series
differences in predictable patterns of change, statistical models. Time series studies can be
they can also be used to identify recurring either univariate or multivariate. However, a
patterns of variation that are confounding time series variable always consists, by defi-
the rate and degree of intervention success. nition, of a series of observations that occur
In contrast with inferential statistical in temporal order. Thus, multivariate time
models, in which aggregate data are general- series analysis is accomplished by identify-
ized to describe changes in human behavior, ing the relationship between or among two
time series analysis uses individual patterns or more pairs of univariate time series.
of change to predict future behavior. Thus, unlike inferential statistical models,
the subject is a unitary entity or system time series data points are not intended to
whose behavioral state can be isolated within be independent of one another. Each value
a given point and measured through a speci- is highly correlated with every successive
fied window of time. Allowing subjects to act value. Thus, any observation in a time series
as their own controls eliminates the random has significantly less individual predictive
heterogeneity of response threat to infer- significance than its inferential counterpart.
ential statistical validity; but limits statisti- In time series analysis, predictive power is
cal external validity. generalization of time not a direct function of sample size. Instead,
series findings requires repeated replications predictive power depends on an accurate
in conceptually congruent others. hypothesis of the internal temporal structure
For the purpose of time series analysis, of the phenomenon, selection of a sampling
the singular system can be defined at many time window of sufficient length to capture
different levels of complexity and inclu- multiple expressions of the change being
siveness. However, because 50 observations studied, and identification of a sampling fre-
across the specified time period is the con- quency that will adequately capture all criti-
ventional minimal number of observations cal phases of the evolving pattern.
necessary for the accurate identification of Although change in behavior is an essen-
predictable patterns of behavior, pragma- tial characteristic of many of the phenomena
tism often limits subjects for time series of interest to nursing science, the use of statis-
nursing research to the often more reliable tical time series models is not always appro-
physiological and directly observed behav- priate or feasible. However, although time
iors of individuals, for example, cardiovas- series analyses are complex and costly, they
cular responses to a cardiac stressor, rather permit nurse scientists to more completely
than equally legitimate, social or behavioral examine and evaluate trends, cycles, and