Choosing a Design
Pseudoexperimental and
True Experimental Designs
Research Designs
 Experimental designs are concerned with the
manipulation of the IV and the measurement of
its effect on the DV
 Descriptive designs are concerned with the
classification of the subjects and with the
application of measurement procedures to
subjects in order to assess group differences,
developmental trends, or relationships among
variables.
Research Designs
Both experimental and descriptive
designs are classified as betweensubjects, within-subjects, or mixed
designs
the selection of a design depends largely
on the research question
Between-Subject Designs
In experimental designs, different groups of
Ss are exposed to different IVs
the IV is applied to one group (exp) but
not to the other group (control)
The major threat to internal validity is subject
selection
differences may be Ss selection differences
rather than the IV
2 ways to equate experimental and control
groups:
randomization
matching
Between-Subject Designs
In descriptive designs, different groups of
Ss are compared with each other with
regard to their performance on some
criterion variable
the important thing with descriptive designs is
to select Ss who fall distinctly into the different
categories of the classification variable but
who are otherwise equivalent with regard to
extraneous variables
classifications must be constructed that are
mutually exclusive
Within-Subjects Designs
The performance of the same subjects is
compared in different conditions.
In experimental research, the subjects are
exposed to all treatment or levels of the
IV
basic concern with these designs is that all
conditions should be equivalent except for the
application of the IV
therefore, it is important to control for
sequencing or order effect
Within-Subjects Designs
There are 2 ways to control for the
sequencing effect:
randomization
presentation of the experimental treatment
conditions (IV) is randomly sequenced
counterbalancing
arrange all possible sequences of treatments (IV)
and then randomly assign subjects to each
sequence
Within-Subjects Designs
In descriptive research, longitudinal
studies would be a within-subjects design
Mixed Designs
One IV may be studied with a betweensubjects design while another IV is
studied with a within-subjects design
this is a mixed design
Research Designs
Two ways to improve generalizability of
findings with research designs:
random sampling of subjects
replication
Types of Experimental
Designs
Pseudoexperimental designs (aka “preexperimental”)
do not have built-in controls
there may be several explanations for the
changes in the DV that are not solely caused
by the IV
all of these designs have uncontrolled
extraneous variables that threaten the
internal validity of the experiment
weaker designs
True experimental designs
Types of Pseudoexperimental
Designs
One-shot case study
One-group pretest-posttest
Static group comparison
One-shot case study
A descriptive study that has big weakness
of no control
no comparisons can be made with this study
statistics: mainly descriptive statistics (e.g.,
means); some inferential statistics (e.g.,
correlation coefficients)
One-group pretest-posttest
Observations are made before and after
the IV has been administered to the group
better than one shot case study, but still
problem with 6 possible uncontrolled
extraneous variables (history, maturation,
testing, instrumentation, statistical
regression, mortality)
statistics: parametric t-test for correlated
samples; nonparametric sign test
Static group comparison
Comparisons are made between one
group which is exposed to the IV and one
group which is not
no pretest to compare posttest scores
no way to ensure equivalence between
groups on relevant extraneous variables
problems may arise from subject selection
variables, testing, instrumentation, mortality
statistics: parametric t-test; Mann-Whitney U
test; chi-square
True Experimental Designs
Pretest-Posttest Control Group
Posttest-Only Control Group
Solomon Four-Group Design
Pretest-Posttest Control
Group
2 groups of Ss are compared on a
measurement or observation on the DV.
Both groups are measured or observed twice
first measurement serves as the pretest and
second measurement serves as the posttest
half of the Ss are randomly assigned to the
first group while the second half are assigned
to second group
Pretest-Posttest Control
Group
Only threats to internal validity are testing
and mortality
statistics: independent t-test to compare
the two groups; analysis of covariance
Posttest-Only Control
Group
Identical to the pretest-posttest control
group design except that the pretest is
not administered to either of the two
groups
by random assignment of Ss to the two
groups, it controls selection, history,
maturation, and statistical regression
testing and instrumentation do no exist since
none of the Ss is measured twice
mortality could be an internal validity
problem
Solomon Four-Group
Design
Subjects are randomly assigned to four
different groups
two of the groups receive the treatment (IV)
two of the groups do not receive the
treatment
only one of the control groups is
administered the pre-test
all four groups are administered the posttest
Solomon Four-Group
Design
It is a combination of the pretest-posttest
and posttest-only control group designs
it controls for all threats to internal
validity
statistics: 2-way ANOVA
What’s Coming Up ….
 Human Subject Certification test
(next week)
 Group Work (next week)
 Specific Group Designs (2 weeks)
 Single-subject Designs (3 weeks)
 Intro/Methods (Oct. 28)
 JOURNAL CLUB (Nov 4)
 Statistics
descriptive
inferential
 Miscellaneous
interpreting research
role of research in CDIS
SLP Journal Club
9 groups
8 groups with 3
members
1 group with 2
members
one group
presentation/week
recommendation:
select article based
on your research
project
AUD Journal Club
2 groups
2 groups with 2
members
one group
presentation/week
recommend select
article from research
project