Experimental Design
True Experimental Designs
Random assignment
 Two comparison groups
 Controls threats to internal validity
 Strongest evidence for cause-effect

Quasi Experimental Designs
Lacks randomization or
 Lacks control group
 Accommodates for natural setting

Group Assignment
Completely randomized (between subjects)
 Randomized block design
 Repeated measures design (within subjects)

Selecting a Design
How many IV are being tested?
 How many levels does each IV have, and
are these levels experimental or control
conditions?
 How many groups of subjects are being
tested?

Selecting a Design
How will subjects be selected, and how will
they be assigned to groups?
 How often will observations of responses be
made?
 What is the temporal sequence of
interventions and measurements?

True Experimental Designs
Single factor designs for independent
groups
 One way design

Design 1A
Pretest-Posttest control group design
 R O1 X O 2


O1
O2
Standard in Clinical research for cause effect
 Randomized controlled trial (RCT)

Design 1B
Two group pretest-posttest design

O 1 X1 O 2
 R O 1 X2 O 2


New vs. old

Difference between 2 treatments
Design 1C
Multigroup pretest-posttest control group
design

O 1 X1 O 2
 R O 1 X2 O 2




O1
O2
Threat to internal validity is attrition
Threat to external validity is the interaction of
treatment and testing
Analysis of Design 1
Utilize change scores
 Interval-ratio data 2 groups unpaired t-test
 3 groups ANOVA
 Ordinal data 2 groups Mann-Whitney U
 3 groups Kruskal-Wallis analysis of
variance

Design 2
Posttest-only design
 R X 1 O2

O2
 True experimental design
 Internal validity is strong
 External validity is strong
 Analysis is identical to Design 1

Multifactor Designs for
Independent Groups
 Design
3 Factorial
 Design
4 Randomized Block
 Design
5 Nested
Design 3 Factorial Design
2
or more Independent variables
 Has factor and levels
2x2
3x3
4x4

Two Way Factorial Design
Main effect
 Interaction effect
 Analysis is two way or
three way analysis of
variance

Design 4: Randomized Block Design
Concern over an extraneous variable
 Attributed variable is crossed with an active
IV
 Analysis is 2 way analysis of variance or
multiple regression analysis

Design 5: Nested Design

Some attribute variables cannot be crossed
Repeated Measures Design
 Design
6 One Way Repeated
Measures Design
 Design
7 Crossover Design
 Design
8 Latin Square Design
Design 6: One Way Repeated Measures
Design
All subjects exposed to all levels of one
treatment variable
 X1 O1
 X2 O2
 X3 O3
 Internal threat is carry over
 Analysis one-way ANOVA

Design 7: Cross Over Design
Minimizes bias of test sequence
 Counterbalances treatment conditions
 Washout period
 R O1 X1 O2 ... O3 X2 O4

O1 X2 O2 ... O3 X1 O4
 Analysis: Paired T-test or 2 way ANOVA
 Ordinal data Wilcoxon signed ranks test

Design 8: Latin Square Design

Sequencing of treatment order is concern

Randomizes treatment order

3 x 3 or 4 x 4
Multifactor Designs for Repeated
Measures

Design 9 Two-way design with two
repeated measures

Design 10 Mixed design
Design 9: Two-way Design with
Two Repeated Measures

Analysis: Two way analysis of variance
with two repeated measures to measure
main and interaction effects
Design 10: Mixed Design

Analysis: 2 way anova with one repeated
measure for main effect two way design
with one repeated factor for interaction
effects
Quasi Experimental Designs
 Do
not use random assignments
 Potential
 Strong
bias of sample
control over design
Quasi Experimental Designs
Design 11 One-group pretest-posttest design
 Design 12 Time series design
 Design 13 Nonequivalent pretest-posttest
control group design
 Design 14 Nonequivalent posttest-only
control group design

Design 11: One-group Pretestposttest Design
 O1 X O2
Threat to internal validity-no comparison
group
 Threat to external validity-potential
interactions with selection
 Analysis: Paired t-test for ratio/internal
 Wilcoxon signed ranks test for ordinal

Multigroup Design

Design 12 Time series design

O1 O2 O3 O4 X O5 O6 O7 O8
Effect of tx on physiological or
psychological variables over time
 IV is time
 Threat to internal validity is history
 Variations 2 or more groups, tx withdrawal

Design 13: Nonequivalent
Pretest-posttest Control Design

O1 X O2
O1 O2
Threat to internal validity is interaction of
selection with history and maturation
 Analysis: Unpairted T-test, anova-interval
or ratio
 Mann-Whitney U-test-Ordinal

Design 14: Nonequivalent PosttestOnly Control Group Design

X O
O
 Threat
to internal validity is
selection bias and attrition
 Analysis:
Regression