Psyc 2030 - Quasi Experiment Lecture.docx

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27 Mar 2012
Quasi-experimental designs
Non equivalent control groups
Time series designs
Program Evaluation
Where program evaluations is helpful
Basic Steps
Quasi experiments
Procedures resemble those of true experiments
But lack the degree of control found in true experiments
Generally occur when the IV involves:
o Subject variables (e.g. personality type)
o An environmental event (e.g hurricane or having
o The passage of time
Hedrick, bickman and Rog, 1993 - " a quasi experimental design is not the method of choice, but
rather a fall back strategy
Cannot infer cause and effect, BUT well designed quasi experiments enable you to demonstrate
that rival interpretations are rendered unlikely
Non-equivalent Group Designs
Posttest only nonequivalent control group
design (aka static group comparison)
X O (treatment group)
O (nonequivalent control group)
Because there is no random assignment to groups, confounding variables may explain any
differences observed
Non equivalent group design
Most common threat to internal validity of this type of design is selection
Pre test / post test non equivalent control group
O x O treatment group
O O non equivalent control group
Pre post
Addition of the pretest measurement allows researcher to compare the observations before
Design allows a researcher to compare the pretest scores and post test scores for both groups
If both groups are similar before intervention but diff after the intervention, research can know
that the intervention was responsible for the change
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