Experiment Basics: Designs
Psych 231: Research
Methods in Psychology

Between-subjects
designs
 Each participant
participates in one and
only one condition of
the experiment.

Within-subjects designs

All participants
participate in all of the
conditions of the
experiment.
Colored
words
participants
Test
BW
words
participants
Colored
words
Test
BW
words
Test
Between vs. Within Subjects Designs
Clock
Colored
words Chair
Cab
participants

Advantages


Test
BW Clock
words
Chair
Cab
Independence of groups (levels of the IV)
Disadvantages

Individual differences between the people in the
groups
• Excessive variability
• Non-Equivalent groups
Between subjects designs

Between-subjects
designs
 Each participant
participates in one and
only one condition of
the experiment.

Within-subjects designs

All participants
participate in all of the
conditions of the
experiment.
Colored
words
participants
Test
participants
Colored
words
Test
BW
words
Test
BW
words
Between vs. Within Subjects Designs

Advantages:



Don’t have to worry about individual differences
Fewer participants are required
Disadvantages


Range effects
Order effects:
• Carry-over effects
• Progressive error
Within subjects designs

Range effects – (context effects) can cause
a problem


The range of values for your levels may impact
performance (typically best performance in
middle of range).
Since all the participants get the full range of
possible values, they may “adapt” their
performance (the DV) to this range.
Within subjects designs

Carry-over effects


Transfer between conditions is possible
Effects may persist from one condition into
another
• e.g. Alcohol vs no alcohol experiment on the effects on
hand-eye coordination. Hard to know how long the
effects of alcohol may persist.
Condition 1
Condition 2
test
Order effects
How long do we
wait for the
effects to wear
off?
test

Progressive error


Practice effects – improvement due to repeated
practice
Fatigue effects – performance deteriorates as
participants get bored, tired, distracted
Order effects

Counterbalancing is probably necessary

This is used to control for “order effects”
• Ideally, use every possible order
• (n!, e.g., AB = 2! = 2 orders; ABC = 3! = 6 orders, ABCD = 4! = 24 orders, etc).

All counterbalancing assumes Symmetrical
Transfer
• The assumption that AB and BA have reverse effects
and thus cancel out in a counterbalanced design
Dealing with order effects

Simple case


Two conditions A & B
Two counterbalanced orders:
• AB
• BA
Colored
words
Test
BW
words
Test
BW
words
Test
Colored
words
Test
participants
Note: this becomes a factorial
design
Counterbalancing

Often it is not practical to use every possible
ordering

Partial counterbalancing
• Latin square designs – a form of partial
counterbalancing, so that each group of trials occur in
each position an equal number of times
Counterbalancing

Example: consider four conditions
Recall: ABCD = 4! = 24 possible orders
1) Unbalanced Latin square: each condition appears
in each position (4 orders)

Order 1
A
B
C
D
Order 2
Order 3
B
C
D
A
C
D
A
B
Order 4
D
A
B
C
Partial counterbalancing

Example: consider four conditions
Recall: ABCD = 4! = 24 possible orders
2) Balanced Latin square: each condition appears
before and after all others (8 orders)

A
B
C
D
A
B
D
C
B
C
D
A
B
C
A
D
C
D
A
B
C
D
B
A
D
A
B
C
D
A
C
B
Partial counterbalancing

Mixed designs


Treat some factors as within-subjects
(participants get all levels of that factor) and
others as between-subjects (each level of
this factor gets a different group of
participants).
This only works with factorial (multi-factor)
designs
Mixed factorial designs

You need to describe:



How many factors
How many levels of each factor
Whether the factors are within or between groups
• e.g., 2 (shallow/deep processing) x 2 (abstract/concrete) mixed
groups factorial design
Describing your design

You need to report:

The main effects
• Depth of processing
• Word Type


Shallow
Deep
abstract
concrete
3.5
4.6
4.1
5.8
The interaction
For each report the means (in the case of the main effects,
report the marginal means) and the statistical outcomes (the
ANOVA results)
• Depth of processing: F(1,226) = 98.6, p < 0.001
• Word type: F(1,226) = 34.0, p < 0.001
• Interaction: F(1,226) = 5.0, p < 0.026


Do this with within complete sentences and paragraphs
Feel free to supplement the text with a graph if it helps with clarity.
Describing your results


Relevant stuff from Ex1
Variables



types, operationalizing
IV: methods of manipulation, getting the right range
DV: measurement
• Validity and Reliability



Sampling
Control
Experimental Designs




Vocabulary
Single factor designs
Between & Within
Factorial designs
Exam 2 Topics (Chpts 4, 6, 11)