Transcript Chapter 11 Experimental Designs

Chapter 11
Experimental Designs
Power Point Slides by Ronald J. Shope in collaboration with John W. Creswell
Key Ideas
• Definition and uses for experimental designs
• Procedures used in experimental research
• Internal and external validity threats in
experimental research
• Types of experimental designs
• Steps in conducting experimental research
• Evaluating experimental research
Educational Research 2e: Creswell
When do you use
experimental procedures?
• To establish cause and effect
– Between independent and dependent
variables
– Control for all variables that might influence
the outcome
• Used when testing theories
• Used when comparing two or more
groups
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Key Characteristics of
Experimental Designs
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Random assignment
Control over extraneous variables
Manipulation of the treatment conditions
Outcome Measures
Group Comparisons
Threats to validity
Educational Research 2e: Creswell
Group comparisons in an
experiment
Phase 1: Relationship Picture
+
Error Correction Treatment
Spelling Accuracy
Test 1
Test 2
6 Weeks 6 Weeks
6 Weeks 6 Weeks
Phase 2: Timeline Picture
Class A: Regular Spelling Practice (Control)
Class B: Reduced word list (Comparison)
Class C: Error Correction (Experimental)
Error Correction Treatment
6 Weeks 6 Weeks
Class C
F-value
9.9
0.27
(3.9)
Phase 3: Statistical Comparisons
Test 1
Test 2
Test 3
Class A
10.3
(3.6)
10.7
(3.3)
11.1
(3.3)
Class B
10.8
(4.3)
Test 3
6 Weeks
6 Weeks
10.6
(3.8)
13.9
(4.2)
4.90*
10.3
(3.6)
13.1
(3.8)
3.31*
Educational Research 2e: Creswell
6 Weeks
* p<.05
Threats to internal validity
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History
Maturation
Regression
Selection
Mortality
Interactions
with selection
• Diffusion of treatments
• Compensatory
equalization
• Compensation rivalry
• Resentful
demoralization
• Testing
• Instrumentation
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Threats to external validity
• Interaction of selection and treatment
• Interaction of setting and treatment
• Interaction of history and treatment
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Types of experiments:
Between groups
• True Experiments
– Pre- and posttest
– Posttest Only
• Quasi Experiments
– Pre
– Posttest
– Factorial Designs
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Means and Main Effects of Eight
groups in Factorial Design
Depression
Low
Health lecture
Type of Instruction
Standard lecture
Medium High
Mean rate Mean rate Mean rate
of
of
of
smoking smoking smoking
Mean rate Mean rate Mean rate
of
of
of
smoking smoking smoking
Main Effects of
Depression
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Main Effects
of Type of
Instruction
Types of Experiments: Withingroup or individual
• Time series experiments
– interrupted
– uninterrupted
• Repeated measures experiments
• Single-subject experiments
– A/B design
– Multiple baseline design
– Alternating treatments
Educational Research 2e: Creswell
Steps in Conducting
Experimental Research
• Decide if an experimental design fits research
problem
• Form cause/effect hypotheses
• Select experimental unit and identify study
participants
• Select an experimental treatment and introduce it
• Choose a type of experimental design
• Conduct the experiment
• Organize and analyze the data
• Develop an experimental research report
Educational Research 2e: Creswell
Criteria for Evaluating
Experimental Research
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Does the experiment have a powerful intervention?
Does it employ few treatment groups (e.g. only two)?
Will participant gain from the intervention(s)?
Is there a systematic way the researcher derived the
number of participants (cell size)?
Was there an adequate number of participants used in
the study?
Were valid, reliable, and sensitive measures or
observations used?
Did the study control for extraneous factors?
Did the researcher control for threats to internal
validity?
Educational Research 2e: Creswell