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count of the number of independent variables we have. Thus, a one-way
within-subjects design contains only a single independent variable, a two-
way within-subjects design contains two independent variables, and so
on. We will explicate the one-way design in this chapter and the two-way
and three-way designs in Chapters 11 and 12, respectively.
Within-subjects designs are also known as
repeated measures designs
because the dependent variable is repeatedly measured, that is, it is mea-
sured under each research condition for each participant. The one-way
within-subjects design is also sometimes called a Treatment
×
Subjects
design because all of the levels of the independent variable (the treatment)
are crossed with (are administered to) all of the subjects (participants) in
the study.
10.3 NATURE OF WITHIN-SUBJECTS VARIABLES
Within-subjects variables are those in which participants are measured
under each of the conditions (i.e., cases are repeatedly measured). Partic-
ipants are thus represented in each and every research condition in the
study. Generally, there are two structural forms that this repeated mea-
surement can take: (a) it can mark the passage of time, or (b) it can be
unrelated to the time of the measurement but simply indicate the condi-
tions under which participants were measured. Although these two forms
of within-subjects designs do not affect either the fundamental nature
of the design or the data analysis, they do imply different research data
collection procedures to create the measurement opportunities.
10.3.1 VARIABLES MARKING TIME
A within-subjects variable marking the passage of time is one in which
the first level of the variable is measured at one point in time, the next
level of the variable is measured at a later point in time, the third level of
the variable is assessed at a still later period of time, and so on. The most
commonly cited example of a time-related within-subjects design is the
pretest-posttest
study. While not a true experimental design (Campbell &
Stanley, 1963; Cook & Campbell, 1979; Shadish, Cook, & Campbell, 2001),
partly because there is no control condition, participants are measured
at least twice, once at the time designated as the pretest and once at
the time designated as the posttest. In such a study the treatment is
ordinarily administered between the two measurements and researchers
wouldordinarilyhavehypothesizedeitheranincreaseoradecreaseinthe
characteristic assessed by the dependent variable from the pretest to the
posttest.
Each measurement (pretest and posttest) is performed in exactly the
same manner by using the same methodology. Thus, there is just one
dependent variable, but individuals are measured at Time 1 and again at
Time 2. In this manner, time completely covaries with the measurement
and it is not uncommon for researchers to talk about the within-subjects
