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?
Design 1A
Pretest-Posttest control group design R O1 X O2 O1 O2
Standard in Clinical research for cause -effect
Randomized controlled trial (RCT)
Design 1B
Two group pretest-posttest design O1 X1 O2 R O1 X2 O2
New vs. old
Difference between 2 treatments
Design 1C
Multigroup pretest-posttest control group design
O1 X1 O2 R O1 X2 O2
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 X1 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
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
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 bias of sample
Strong 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 Pretest-posttest 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