Transcript Quasi-Experiments

QUASI-EXPERIMENTS
What is a Quasi-Experiment? What is a Single-Group Time
Series Design?
Why Do Quasi-Experiments?
What is an Ex Post Facto
Design?
What is a Single-Group
Posttest Only Design?
What is a Single-Group
Pretest/Posttest Design?
What is a Nonequivalent
Control Group
Pretest/Posttest Design?
What is a Multiple-Group
Time Series Design?
What is a Cross-Sectional
Design?
What is a Longitudinal
Design?
What is a Sequential or
Cohort Design?
How Are Quasi-Experiments
Analyzed?
What is a Quasi-Experiment?
Compare subjects in different conditions
on a DV
w Lacks one or more criteria for an
experiment (cause, comparison, control)
w Interpreted like a correlational study
w
Why Do Quasi-Experiments?
Internal validity is a problem because the
IV is not manipulated
w It may not be possible or ethical to
manipulate the IV
w
What is an
Ex Post Facto Design?
Compare groups that differ on a preexisting variable (subject variable)
w Examples:
w
• gender
• personality
• presence of mental disorder
What is a Single-Group
Posttest Only Design?
Scores are measured after a treatment for
one group
w The treatment is not manipulated, and there
is no comparison
w
What is a Single-Group
Pretest-Posttest Design?
Scores are measured before and after an
event or treatment
w The treatment is not manipulated
w
What is a Non-Equivalent
Control Group
Pretest/Posttest Design?
Compare a treatment group and a control
group before and after the treatment
w The groups are not randomly assigned
w
What is a Single-Group
Time Series Design?
Scores are measured several times before
and after an event or treatment
w Better than Pretest-Posttest design, because
you can tell whether the change is likely to
be due to a random fluctuation
w
Fig 1 Suicides in Toronto by jumping or other means before (1993-2001) and after (July
2003-June 2007) construction of a suicide prevention barrier at Bloor Street Viaduct:
corrected per capita to suicides in 1993 population (not standardised for age)
Sinyor, M. et al. BMJ 2010;341:c2884
Copyright ©2010 BMJ Publishing Group Ltd.
What is a Multiple-Group
Time Series Design?
Scores for a treatment and control group
are measured several times before and after
w The control group is not randomly assigned
w Combination of Nonequivalent Control
Group and Interrupted Time Series
w
What is a
Cross-Sectional Design?
Compare different age groups at one point
in time
w The groups may differ on other variables,
since age is not manipulated
w cohort effects - generational history
differences
w
What is a
Longitudinal Design?
w
w
w
Measure the same group at different ages
Time-consuming
Attrition can be a problem
What is a Sequential
or Cohort Design?
Compare different age groups at different
ages
w Combination of longitudinal and crosssectional
w
How Are Quasi-Experiments
Analyzed?
Choose from the same set of statistics used
for experiments
w Remember that you interpret differences
like the study was correlational
w Take into account number of conditions
and whether variables are between or within
subjects
w