This document discusses different research design strategies. Weak designs do not control for extraneous variables and have threats to internal validity. Strong designs use control groups and techniques to eliminate rival hypotheses. Control groups allow comparison to estimate the effect of a treatment by controlling for other influences. Random assignment creates equivalent groups and controls for threats to internal validity. Pretest-posttest designs with control groups allow comparison of change scores and ensure initial group equivalence.

1. Chapter 8

2. 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.

3. Experimental Research Design
• Weak vs. Strong experimental design

4. Weak Experimental Designs
 Do not control for many extraneous variables
 Provide weak evidence of cause and effect
 Have threats to internal validity

5. One-group Posttest- Only
Design
 Influence of a treatment is investigated with
only one group of individuals
 Single test after treatment (“posttest”)
 Example: New reading program in 1st grade
 Does program improve reading skills?

6. One-group Posttest-only
Design
Treatment Posttest Measure
X O

7.  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.

8. 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

9. 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

10. Nonequivalent Posttest-Only
Design
 Performance of an experimental group is
compared with that of a nonequivalent control
group at posttest
Posttest
Treatment Measure
Experimental Group X O Compare
Control Group O

11.  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

12. Strong Experimental Research
Designs
 Designs that effectively control extraneous
variables and provide strong evidence of
cause and effect

13.  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

14.  Control group
 comparison group
 Does not receive “active” level of IV
○ No intervention or standard experience
 Experimental group
 receives a level of the IV intended to produce
effect

15. 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

16. Functions of a Control Group
2. Control for rival hypotheses
 With random assignment, extraneous variables
will have an equivalent impact on both groups

17. Strong Experimental Research
Designs
 Basic designs – one IV and one DV
 Between-participants
 Within-Participants (repeated measures)
 Factorial Designs – multiple IVs

18. Posttest-Only Control Group
Design
 This design looks familiar, right?
 What is different now?
Posttest
Treatment Measure
Experimental Group X O Compare
Control Group O

19. Posttest-Only Control Group
Design
 We could have more than 1 experimental
group
Posttest
Treatment Measure
Control Group O
Experimental Group 1 X1 O Compare
Experimental Group 2 X2 O

20. Important time-out:
 Benefits of randomized control group
 Avoid many threats to internal validity
 Examples?

21. Pretest-Posttest Control Group
Design
 Simply add pretest to previous design
 What comparisons will we make?
Pretest Posttest
Treatment
Measure Measure
Experimental
O X O
Group
Control
O O
Group

22. 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