chapter 8 part 1(1)
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EXPERIMENTAL RESEARCH DESIGN
Chapter 8
Research Design
The plan or strategy used to answer the research question
This is where we put to use all of the things that we have learned about validity and reliability.
Experimental Research Design• Weak vs. Strong experimental design
Weak Experimental Designs
Do not control for many extraneous variables Provide weak evidence of cause and effect Have threats to internal validity
One-group Posttest- Only Design
Influence of a treatment is investigated with only one group of individualsSingle test after treatment (“posttest”)
Example: New reading program in 1st gradeDoes program improve reading skills?
One-group Posttest-only Design
Treatment Posttest Measure
X O
Problems? Extraneous variables? Why/when would you use this design? If there is background information on DV and prior
work has shown mechanism by which IV impact DV.
One-group Pretest-Posttest Design
A treatment condition is interjected between pre- and posttest of the dependent variable.
Pretest measure Treatment Posttest Measure
O X O
Compare
One-group Pretest-Posttest Design
Advantage over one-group posttest? Problems? Threats to internal validity? Why would we use this design?
If we cannot create an equal group for comparison, we can get SOME information this way
Nonequivalent Posttest-Only Design
Performance of an experimental group is compared with that of a nonequivalent control group at posttest
TreatmentPosttest Measure
Experimental Group X O
Control Group OCompare
Advantage over previous designs? Problems? Threat to internal validity?
Selection Best thing to do is to use random assignment.
Creates equivalent groups Next best is to use matching on relevant variables.
Will discuss this next chapter
Strong Experimental Research Designs Designs that effectively control extraneous
variables and provide strong evidence of cause and effect
To control for extraneous variables you must eliminate potential rival hypotheses.
Done in two ways:Control Techniques- more on this next week (ch. 7)Control Group
Control groupcomparison groupDoes not receive “active” level of IV
○ No intervention or standard experience
Experimental groupreceives a level of the IV intended to produce
effect
Functions of a Control Group
1. Serve as a comparison to the experimental group. Did the treatment/manipulation have an effect?
Estimate Counterfactual- what the participants’ responses would have been had they not received the treatment
Functions of a Control Group2. Control for rival hypotheses
With random assignment, extraneous variables will have an equivalent impact on both groups
Strong Experimental Research Designs Basic designs – one IV and one DV
Between-participantsWithin-Participants (repeated measures)
Factorial Designs – multiple IVs
Posttest-Only Control Group Design
This design looks familiar, right? What is different now?
TreatmentPosttest Measure
Experimental Group X O
Control Group OCompare
Posttest-Only Control Group Design
We could have more than 1 experimental group
TreatmentPosttest Measure
Control Group O
Experimental Group 1 X1 O
Experimental Group 2 X2 OCompare
Important time-out:
Benefits of randomized control group Avoid many threats to internal validity
Examples?
Pretest-Posttest Control Group Design Simply add pretest to previous design What comparisons will we make?
PretestMeasure Treatment Posttest
Measure
Experimental Group O X O
Control Group O O
Benefits of Pretest
Ensure equivalency of groups Detect ceiling and floor effects
Select participants accordingly Can empirically demonstrate effect of treatment
Change scores See if initial position on DV is important
Treatment may affect hi/low differently