Chapter 10
Extending the Logic
of Experimentation:
Within-Subjects and
Matched-Subjects
Approaches
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Topics
1. Within-Subjects Designs
2. Mixed Designs
3. Matched-Subjects Designs
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Within-Subjects Designs
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Within-Subjects Designs
• The participant’s own performance is the basis
of comparison
• Compare the performances of the same set of
participants on the dependent variable
following different treatments
• Let’s look at an experiment
– First as a between-subjects experiment
– Then as a within-subjects experiment
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Within-Subjects Designs (cont’d.)
• If we identify the individual participants, the
design is:
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Within-Subjects Designs (cont’d.)
• We can perform this same study as a
within-subjects design
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Table 10.1 Analysis of Variance F Table for the Basketball Study
as a Between-Subjects Design and as a Within-Subjects Design
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Within-Subjects Designs (cont’d.)
• Potential problem in basketball study
– The results from the no-feedback condition might
have a potential carryover effect on the feed-back
condition
• To control for this potential problem:
– Use a counterbalancing procedure
• Intrasubject counterbalancing
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An Illustration of
Within-Subjects Research
• Interaction effect
– Crashes were caused by a visual illusion but only
when the pilots were distracted from instruments
• Perform a simple 2 X 3 X 2 design
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An Illustration of
Within-Subjects Research (cont’d.)
• Dependent variable: amount of landing error
• Using a between-subjects design
– Randomly assign pilots to one of the four possible
conditions (A1B1, A1B2, A2B1, A2B2)
• Is a between-subjects design the best
alternative?
• Use a within-subjects design
– Each cell is composed of the same participants
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Within-Subjects Designs
• Advantages of within-subjects designs:
– Ensures that all groups are equal on every factor
at the beginning of the experiment
– The total number of research participants can be
reduced dramatically
– Statistically more sensitive to changes in the
treatment effect
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Within-Subjects Designs (cont’d.)
• Disadvantages of within-subjects design
– Within-subjects design is not appropriate when:
• The treatment has a lasting effect or
• The purpose of the study is to test for a lasting effect
– Extremely sensitive to time-related effects
• Order effects: effects brought about through
continued repetition of the tasks
– Fatigue effects: a decline in performance
– Practice effects: an improvement in performance
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Counterbalancing
• Conditions for complete counterbalancing:
– Each condition must occur equally often
– Each condition must precede and follow all other
conditions an equal number of times
• Intragroup counterbalancing: ensures that
every possible sequence appears at each
presentation of the treatment
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Counterbalancing (cont’d.)
• Latin square design
– Special case of incomplete counterbalancing
– Common use: psychopharmacological research
• Counterbalancing will not control for a
differential order effect
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Repeated Measures
• One common application of within-subjects
designs
– Repeated measures as one factor in a factorial
design
– Useful in studying psychological processes that
occur over time
– Widely used in studying human and animal
learning processes
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Mixed Designs
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Mixed Designs
• Designs that include both “within” and
“between” components
• Example:
– Assume that we want to perform a biofeedback
experiment to determine the effect of feedback
on our ability to control heart rate
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Mixed Designs (cont’d.)
• Such an experiment is diagrammed as:
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Mixed Designs (cont’d.)
Figure 10.1
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Matched-Subjects Designs
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Matched Subjects Design
• By using a matched-subjects design, we:
– Reap some of the advantages of within-subjects
designs and
– Take advantage of the random assignment of
participants that is possible with a betweensubjects design
• Matching can be used in either of two ways:
– As a control procedure
– As an experimental procedure
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Matching as a Control Procedure
• When a particular individual variable or
characteristic has a high correlation with the
dependent variable
– Equal groups may be obtained by matching along
this characteristic
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Matching as a Control Procedure
(cont’d.)
• Two steps in forming matched groups of
participants:
– Pairs of participants are matched on some
measure that is correlated with performance on
the dependent variable
– One member of each pair is assigned randomly to
either the experimental or the control group; the
other member is then assigned to the other group
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Figure 10.2 The six major steps
in a matched-subjects design
used to study changes in brain
chemistry resulting from
prolonged sensory deprivation.
(1) Rank order all subjects on
the aspect of blood chemistry
that is known to be correlated
with brain neurochemistry. (2)
Form pairs of subjects on the
basis of this rank order. (3)
Randomly assign one member
of each pair to the experimental
group and one member to the
control group. (4) Conduct the
experimental treatments. (5)
Conduct neurochemical
analyses. (6) Compare the
results for the experimental and
control animals. Note that,
except for our ranking and
matching procedures, the
design is similar to the
completely randomized designs
discussed previously
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Matching as an
Experimental Procedure
• Randomized block design: resulting procedure
when the matching factor is analyzed
• Time estimation study factorial design
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Matching as an
Experimental Procedure (cont’d.)
• Advantages resulting from the prior matching
of participants to groups
– Knowledge gained from the interaction effect
– Matching ensures that the groups in a study are
equal before the treatment is introduced
– Matching participants reduces the within-groups
variance (error variance)
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Terminology
• Glass and Stanley (1970) suggest:
– Reserve the term blocking for cases in which the
matching takes place on a nominal-scale factor
– The use of twins be considered matching on a
nominal-scale factor
– When the matching uses ordinal measurements,
use the term stratifying
– When interval or ratio scales are used, use the
term leveling
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Summary
• Within-subjects designs: every participant
serves in every group and receives all levels of
the independent variable
• Mixed design: combines both “within” and
“between” designs
• Matched-subjects procedure may be used
either as a control procedure or as an
experimental procedure
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