Experimental and Quasi-experimental Designs

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```					Experimental and Quasi-
experimental Designs

Having fun with X’s and O’s
Inferring Causality

   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
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:
RE    O1    X   O2
RC    O1        O2
How a Classic Experiment Works (p. 324)

Experimental Group                  Control Group
Compare:   Measure Dependent
Measure Dependent
Variable           Same?          Variable

The Intervention

Remeasure         Compare:        Remeasure
Dependent Variable                Dependent Variable
Different?
Distinguishing Characteristic of
Experimental Designs
   Randomization: comparability of groups
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
Quasi-experimental Designs

   Classic Experimental Design, without
Randomization

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

   Use Multiple Pretests
O1   O2   O3    X       O4   O5   O6
Beefing Up Quasi-Experiments

   Vary the timing of the intervention

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

   Use non-equivalent interventions

E    O1    X1   O2
C    O1    X2   O2
Beefing Up Quasi-Experiments

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

EM   O1    X    O2
CM   O1         O2

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