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CHAPTER SIX
One-Way Between-Subjects Design
6.1 OVERVIEW
In a between-subjects design the levels of the independent variable are
represented by different participants. A one-way between-subjects design
has just one independent variable. In the room color study described in
earlier chapters, for example, the independent variable was the color of the
room. The independent variable is said to have levels, which are usually
referred to as treatment levels or treatment conditions. A level is the value
of the independent variable for a given group. Our room color variable
had two levels, red and blue. In the data file, these conditions would be
coded arbitrarily as 1 and 2.
We are not limited to having only two levels for an independent vari-
able. Theoretically, we could have a very large number. Practically, how-
ever, half a dozen or so levels are about as many as you will ordinarily
see. In the example that we will use in this chapter, we will have five lev-
els. Because this is a between-subjects design, each participant is studied
under just one of these conditions and contributes just a single score to
the data analysis.
6.2 A NUMERICAL EXAMPLE
The SAT is used by a wide range of colleges and universities as part of the
application process for college admission. Assume that we are interested
in the effect of preparation time on SAT performance. Because it would
be unethical to have assigned students randomly to the groups in this
particular study, assume that we opted for a quasiexperimental design
(the classic descriptions of quasiexperimental and nonexperimental have
been presented by Donald T. Campbell and his associates: Campbell,
1957; Campbell & Stanley, 1966; Cook & Campbell, 1979; Shadish, Cook,
& Campbell, 2002), where we have found samples of students who have
studied for the durations specified by our group structure. In this manner,
we have generated five groups of students with each group being composed
of seven cases. The groups contain students who have studied for either
zero, two, four, six, or eight months prior to taking the SAT.
Our dependent variable is the average of the Verbal and Quantitative
SAT scores. The standardized mean on the SAT is 500 with a standard
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