Quasiexperimental Designs, Experimental Designs, and SingleSubject Designs - PowerPoint PPT Presentation

1 / 24
About This Presentation
Title:

Quasiexperimental Designs, Experimental Designs, and SingleSubject Designs

Description:

Instrumentation. Statistical Regression. Selection. Ambiguity about ... Instrumentation - the effect of instrumentation with poor reliability on measures ... – PowerPoint PPT presentation

Number of Views:207
Avg rating:3.0/5.0
Slides: 25
Provided by: laurak9
Category:

less

Transcript and Presenter's Notes

Title: Quasiexperimental Designs, Experimental Designs, and SingleSubject Designs


1
Quasi-experimental Designs, Experimental Designs,
and Single-Subject Designs
  • OT 667
  • July 2, 2003

2
Experimental Design
  • Control Group
  • Random Assignment
  • Manipulate a variable
  • (treat one group but not the other, OR use 2
    different treatments)

3
Random Assignment
  • Subjects have an equal change of being assigned
    to the control or treatment group
  • Assures group equivalence

4
Manipulation of Variables
  • Active variable one that is manipulated by the
    researcher
  • Attribute variable traits of subjects, such as
    age, diagnosis, gender, etc cannot be
    manipulated by the researcher
  • Can manipulate one or more active variables at
    the same time
  • Can also assign people with various attribute
    levels to receive the active variable

5
Blinding in Research
The purpose of blinding is to minimize bias in
the process of recording or collecting
information about the outcome of the treatment.
6
So who gets blinded?
  • Subjects
  • Persons administering the treatment
  • Persons collecting the data
  • Persons analyzing the data

7
Quasi-experimental Research
8
Quasi-experimental designs are those studies
which do not use - random assignment - a
control group
9
Rationale for Using Quasi-Experimental Design
  • Clinical limitations space, time, money,
    equipment/personnel needed to provide
    treatments.
  • Use of quasi-experimental design can be a step to
    collect data to apply for funding for
    experimental design
  • Ethical issues random assignment and use of a
    control group are not always acceptable to the
    population or sample under study

10
Kinds of Quasi-Experimental Designs
  • One group pretest-posttest design
  • Time Series design
  • Nonequivalent pretest-posttest design
  • Nonequivalent posttest only design

11
Comparison of Nonequivalent Pretest-Posttest
Control Group Designs and Nonequivalent
Pretest-Posttest Designs
  • Nonrandom selection
  • Control group
  • Baseline measures and post-intervention measures
    collected
  • Nonrandom selection
  • No control group
  • Baseline measures and post-interventions measures
    collected

12
Nonequivalent Posttest-Only Control Group Design
  • Two groups involved which are selected, not
    randomly assigned
  • Intervention administered
  • Posttest measures taken on the variable under
    study

13
The definition of time-series designs in your
text is the same as single subject, classifying
this design as quasi-experimental. Not everyone
agrees with this classification.
14
Does the experimental treatment really cause the
observed change in the dependent variable?This
question addresses the internal validity of a
study.
15
Threats to Internal Validity
  • History
  • Maturation
  • Attrition
  • Testing
  • Instrumentation
  • Statistical Regression
  • Selection
  • Ambiguity about Direction of Causal Influence
  • Treatment Diffusion
  • Demoralization of respondents receiving treatment
    assumed to be less desirable

16
Threats to Internal Validity
  • History - events occurring after administration
    of the independent variable that could affect or
    cause outcomes
  • Maturation - changes in behavior/skills occurring
    over time after administration of the independent
    variable
  • Attrition - subjects who drop out of the study,
    causing groups to be asymmetrical

17
Threats to Internal Validity
  • Testing - the effects of having repeated tests on
    behaviors under study OR the effect of certain
    kinds of assessment on subjects behavior
  • Instrumentation - the effect of instrumentation
    with poor reliability on measures
  • Statistical regression - another reliability
    concern extreme scores on a given variable can
    change to approach the mean over time (regression
    to the mean) a real issue when subjects are
    chosen because of extreme scores on a variable

18
Threats to Internal Validity
  • Selection - When subjects are selected for groups
    rather than randomly assigned, the groups end up
    being different. Matching can help but does not
    balance out the selection factor completely
  • Diffusion - occurs when one group becomes aware
    of the treatment used for the other group and
    integrates some aspect of the treatment into
    their own intervention
  • Compensatory Equalization - when persons
    delivering one intervention believe the treatment
    the other group receives is more desirable and
    works harder, ultimately delivering compensatory
    services

19
Threats to Internal Validity
  • Compensatory rivalry and resentful demoralization
    - this is the situation when the participants in
    one group believe the other groups treatment is
    desirable and work extra hard to compensate,
    thereby affecting the outcomes. Resentful
    demoralization occurs when one group becomes
    angry with the treatment they receive and dont
    work as hard as they should
  • Ambiguity about causal influence - when the
    cause-effect sequence is unclear - does the
    independent variable cause the dependent variable
    or vice versa?

20
Threats to Internal Validity
  • Selection and history - when the history of
    non-randomized subjects interacts with selection
    to influence the outcomes
  • Subjects and maturation - when selected subjects
    in experimental and control groups mature at
    different rates, affecting the outcomes
  • Subjects and instrumentation - when the
    instrumentation results in different measures for
    each group involved in the study

21
WHEW!
22
Can the results of the study be generalized to
the population of whom the sample is
representative in a given study? This question
asks about the external validity of a study.
23
Threats to External Validity
  • Interaction of Treatment and Selection
  • Interaction of Treatment and Setting
  • Interaction of Treatment and History

24
Your challenge in all future empirical readings
(experimental and quasi-experimental) is to
consider threats to internal validity and discard
threats if at all possible.
Write a Comment
User Comments (0)
About PowerShow.com