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CH 10 Experimental Research- often…
CH 10 Experimental Research- often considered to be the ‘gold standard’ in research designs is one of the most rigorous of all research designs
one or more independent variables
are manipulated by the researcher (as treatments), subjects are randomly assigned to different
treatment levels (random assignment), and the results of the treatments on outcomes (dependent
variables) are observed
internal validity
(causality) due to its ability to link cause and effect through treatment manipulation, while
controlling for the spurious effect of extraneous variable.
Laboratory experiments, conducted in laboratory (artificial) settings,
tend to be high in internal validity, but this comes at the cost of low external validity
(generalisability), because the artificial (laboratory) setting in which the study is conducted may
not reflect the real world
Field experiments are conducted in field settings such as in a real
organisation, and are high in both internal and external validity. But such experiments are
relatively rare, because of the difficulties associated with manipulating treatments and controlling
for extraneous effects in a field setting
BASIC CONCEPTS
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Random selection and assignment. Random selection is the process of randomly drawing a
sample from a population or a sampling frame
standard practice in true experimental research to
ensure that treatment groups are similar (equivalent) to each other and to the control group
prior to treatment administration
Threats to internal validity. Although experimental designs are considered more rigorous
than other research methods in terms of the internal validity of their inferences (by virtue of their
ability to control causes through treatment manipulation)
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Two-group experimental designs
The simplest true experimental designs are two group designs involving one treatment group and
one control group
Pretest-posttest control group design : subjects are randomly assigned to treatment and control groups, subjected to an initial (pretest) measurement of the dependent variables of interest, the treatment group is administered a treatment (representing the independent variable of interest), and the dependent variables measured again (posttest)
Posttest-only control group design. This design is a simpler version of the pretest-posttest
design where pretest measurements are omitted
Covariates are those variables that are not of central
interest to an experimental study, but should nevertheless be controlled in an experimental design
in order to eliminate their potential effect on the dependent variable and therefore allow for a
more accurate detection of the effects of the independent variables of interest
Factorial designs
Two-group designs are inadequate if your research requires manipulation of two or more
independent variables (treatments)
main effect is said to exist if the dependent variable shows a significant
difference between multiple levels of one factor, at all levels of other factors
interaction effect exists when the effect of differences in one
factor depends upon the level of a second factor.
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Quasi-experimental designs are almost identical to true experimental designs, but lacking one key
ingredient: random assignment
quasi-equivalent version of pretest-posttest control group
design is called non-equivalent groups design
quasi experimental version of switched replication design is called non-equivalent switched
replication design
Regression discontinuity (RD) design. This is a non-equivalent pretest-posttest design where
subjects are assigned to the treatment or control group based on a cut-off score on a preprogram
measure
Proxy pretest design. This design, shown in Figure 10.11, looks very similar to the standard
NEGD (pretest-posttest) design, with one critical difference: the pretest score is collected after the
treatment is administered
Separate pretest-posttest samples design. This design is useful if it is not possible to collect
pretest and posttest data from the same subjects for some reason.