Within Subjects (Participants) Designs
All participants serve in each condition of the
study.
Advantage: Individual Differences are not a
Confound.
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Types of WS Designs
1) Pre-Post Design with no treatment control
group.
Used when the purpose is show a change
between conditions (i.e. treatment or an
outcome) and the order of the conditions is
logically set (i.e., pre test must proceed post
test).
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2) WS design with counterbalancing for
order.
Used when comparing conditions and the
conditions can be measured in any order.
Weakness: Sequencing Effects:
Participation in one condition may effects
participation in a subsequent condition(s).
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A) Order Effects
Changes in performance due to practice,
familiarity, boredom that are due to repeated
testing and not to differences in the IV.
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B) Carry-over Effects Long-lasting effects
participation in one condition that carry-over
to effect performance in subsequent
conditions.
e.g., Learning, Expectation, attitude, physical
changes due to experimental condition.
E.g., Reward Contrast effects in Rats
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Example of Carry Over effects: Reward
Contrast effects in Rats
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Controlling for Sequence Effects (WS Design)
1) Counterbalance - controls for order effects
and for unexpected carry-over effects.
Counterbalancing: Within your study, each
condition occurs equally as often in each
ordinal position, and each condition occurs
equally as often before and after each other
condition.
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Randomized Counterbalancing
Each Subject gets a different order.
ABC
ACB
BCA
BAC
CAB
CBA
Subjects are randomly assigned to order
conditions.
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Complete Randomized
Counterbalancing
All possible orders of conditions are used in
and participants are randomly assigned so that
there are equal numbers in each order
condition.
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Equal numbers of subjects are assigned to each order
condition, thus order effects are controlled by
equalling the effects of order for each condition.
Condition A
Condition B
Condition C
1st
2nd
3rd
1st
3rd
2nd
2nd
3rd
1st
2nd
1st
3rd
3rd
2nd
1st
3rd
1st
2nd
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Incomplete Counterbalancing
With 4 conditions all orders would total 24.
ABCD
BACD
CABD
DABC
ABDC
BADC
CADB
DACB
ACBD
BCAD
CBAD
DBAC
ACDB
BCDA
CBDA
DBCA
ADAB
BDAC
CDAB
DCAB
ADBA
BDCA
CDBA
DCBA
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Rules for Incomplete Counterbalancing
1) Each condition must appear an equal
number of times in each ordinal position.
2) Each condition must precede and be
followed by every other condition equal
numbers of times.
A
B
D
C
B
C
A
D
C
D
B
C
D
A
C
B
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Intrasubject counterbalancing
Each subject gets each possible order.
ABCCBA
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Between Subjects Designs
– Each participant serves in only one
condition in the study.
Confound: Individual Differences
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Control of Individual Differences e.g., IQ,
personality, sex, race, age etc.
Random Assignment - Each member of the
sample has an equal chance of being assigned
to any of the conditions.
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Power
Ability to find a difference
between conditions when a
difference actually exists.
When designing a study we
want high power!!
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Power and Designs
1) WS designs have higher power than BS
designs and should ALWAYS be
considered first.
ASK
Can I counterbalance for order?
If yes, then counterbalance.
If no, use pre-post test with no-treatment
control group.
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BS designs are used only when participants
cannot be in more than one condition of the
study.
• Being in each condition permanently
changes a person so that they cannot be in
other conditions of the study.
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Pre
Post
Change
Cognitive Therapy
Drug treatment
Combined Treatment
No treatment Control
Comparison between treatment types here is a
BS comparison
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BS designs are commonly used for Natural
Manipulation studies (e.g., sex differences,
personality differences). These are not
experiments and they have low Internal
Validity.
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