Quasi-Experimental
Designs
Passer Chapter 11
Slides Prepared by Alison L. O’Malley
Quasi-Experimentation
• Quasi-experiments resemble experiments, but
lack experimental control
• Generally, lack of random assignment is the key
point of distinction between quasi-experiments
and “true” experiments (Shadish, Cook, &
Campbell, 2002)
• Quasi-experiments are thus more vulnerable to
internal validity threats
If quasi-experiments lack experimental control,
what good are they?
Quasi-Experimentation:
Designs without a control group
•One-group posttest-only design
• A treatment occurs and the DV is measured
afterward
What threats to interval validity are present
here?
Quasi-Experimentation:
Designs without a control group
•One-group pretest-posttest design
• DV measured before and after treatment
Quasi-Experimentation:
Designs without a control group
•Simple interrupted time-series design
• DV repeatedly measured before and after a
treatment
• History is the primary threat to internal
validity
Quasi-Experimentation:
Designs with a nonequivalent control group
• Selection emerges as a major threat to
internal validity
• Selection may interact with other threats (i.e.,
selection interactions)
• In such cases, the threat is labeled by
replacing the term “selection” with
“differential” (e.g., differential attrition,
differential testing)
Quasi-Experimentation:
Posttest only with nonequivalent control
group
•Participants in one condition exposed to a
treatment
•Participants in the other nonequivalent
condition are not exposed to the treatment
•Outcome measures obtained from both
groups
•Lack of pretests poses difficulties in
interpreting results
Quasi-Experimentation:
Pretest-posttest with nonequivalent control
group
•Pretreatment and posttreatment scores
are obtained for a treatment group and a
nonequivalent control group
•What benefits are added by this
research approach?
Quasi-Experimentation:
Simple interrupted time-series with
nonequivalent control group
•A series of pre- and posttreatment scores are
obtained for a treatment group and a
nonequivalent control group
Quasi-Experimentation:
Simple interrupted time-series with
nonequivalent control group
•What information can be obtained from
examining the pretreatment trend lines?
Switching Replication Designs
• One group receives a treatment while a
nonequivalent group does not receive a
treatment but is then exposed to treatment
down the road
• Can be used with both pretest-posttest and
time-series designs
• In the switching replication with treatment
removal, the initial treatment group no longer
receives the treatment once the control group is
switched
Switching Replication with
Treatment Removal
How might you improve the design of this study?
Program Evaluation
• Assesses the need for as well as the
design, implementation, and effectiveness
of a social intervention
• What is a recent social intervention
undertaken in your community?
Program Evaluation
• Much talk surrounds “evidence-based”
programs and public policies
• How do you know whether a program or
policy works?
Program Evaluation: Needs Assessment
• Needs assessment determines whether
there is a need for a social program, and if
so, what is required to meet the need
• Must acquire data from a wide range of
sources
Program Evaluation:
Program Theory and Design Assessment
• Rationale for designing a program in a
particular way – theoretical and empirical
justification
Program Evaluation:
Process Evaluation
• Is program implemented as intended?
• Also known as program monitoring
Program Evaluation:
Outcome Evaluation
• Likely more comfortable terrain, as this
deals with assessing program (treatment)
effectiveness
• If randomized controlled trials aren’t
possible, turn to alternative designs
Program Evaluation:
Outcome Evaluation
• Watch out for contamination, which
occurs when knowledge, services, or other
experiences intended for one group are
unintentionally received by another group
Program Evaluation:
Efficiency Assessment
• Cost-benefit analysis of program
effectiveness
• Is the program financially beneficial?
Program Evaluation:
Program Diffusion
• Implementing and maintaining effective
programs in other settings or with other
groups
Dissemination
Adoption
Implementation
Sustainability