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CHAPTER TEN
One-Way Within-Subjects Design
10.1 THE CONCEPT OF WITHIN-SUBJECTS VARIANCE
In between-subjects designs, participants contribute a single value of the
dependent variable, and different participants are represented under each
level of the independent variable. The situation is different for within-
subjects designs. Here, we register a value of the dependent variable in
each and every condition of the study for every one of the participants or
cases in the study. That is, each participant is measured under each level
of the independent variable.
Within-subjects designs are still univariate studies in that there is only
one dependent variable in the design. As is true for between-subjects
designs, the dependent variable is operationalized in a particular manner.
What differentiates between-subjects and within-subjects designs is the
number of such measurements in the data file that are associated with
each participant: In between-subjects designs, researchers record just one
data point (measurement) for each participant; in within-subjects designs,
researchers record as many measurements for each participant as there are
conditions in the study.
Within-subjects designs can theoretically contain any number of inde-
pendent variables combined in a factorial manner. Practically, because all
participants must contribute data under each level of each independent
variable, the logistics of the data collection procedures can become quite
challenging with several independent variables in the design. Also, there
is the possibility that subjects could be affected or changed by exposure
to one level of an independent variable, a situation known as
carry-over
effects,
such that it may not be appropriate to expose them to more than
that single level; the likelihood of eliminating all of the possible carry-over
effects diminishes with each additional independent variable included
in the design (we will discuss carry-over effects more in Section 10.4).
For these reasons, it is relatively uncommon to encounter within-subjects
designs incorporating more than three independent variables.
10.2 NOMENCLATURE
Within-subjects designs are named by using the same strategy as is used
for between-subjects designs: we talk about
n
-way designs where
n
is a
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