Experimental and Quasi-experimental Designs

1
Experimental and Quasi-experimental Designs

• Having fun with Xs and Os

2
Inferring Causality

• The essence of evaluation
• Is the effect the result of the cause?
• Reality rarely accommodates us
• The mischief of intervening variables threats
  to internal validity
• The year the Twins won the pennant
• Other threats to internal validity p. 317, 328

3
Experimental Designs

• The language of experimental design
• X intervention
• O Observation
• O1 Sequence of observation
• R Randomization
• E Experimental Group C Control
• The Gold Standard

4
How a Classic Experiment Works (p. 324)
Experimental Group
Control Group
Compare Same?
Measure Dependent Variable
Measure Dependent Variable
The Intervention
Compare Different?
Remeasure Dependent Variable
Remeasure Dependent Variable
5
Distinguishing Characteristic of Experimental
Designs

• Randomization comparability of groups
• What is the ethical problem?
• Possible solution matching
• Similar to stratification of a sample

6
Pre-Experimental Designs

• The Post-test only, or one-shot design
• X O1
• The One-Group Pretest-Posttest Design
• O1 X O2
• Posttest only with nonequivalent groups (p. 322)

X O1 O2
7
Quasi-experimental Designs

• Classic Experimental Design, without
  Randomization

8
Beefing Up Quasi-Experiments

• Use Multiple Pretests

O1 O2 O3 X O4 O5 O6
9
Beefing Up Quasi-Experiments

• Vary the timing of the intervention

E O1 X O2 O3 C O1 O2 X O3
10
Beefing Up Quasi-Experiments

• Use non-equivalent interventions

11
Beefing Up Quasi-Experiments

• Select a Control group that matches the
  experimentals on variables of interest