designs fair better than pre-experimental studies in
that they employ a means to compare groups.
They fall short, however on one very
important aspect of the experiment: randomization.
Posttest Nonequivalent Group.
With this design, both a control group and an
experimental group is compared, however, the groups
are chosen and assigned out of convenience rather
than through randomization.
This might be the method of choice for our
study on work experience as it would be difficult to
choose students in a college setting at random and
place them in specific groups and classes.
We might ask students to participate in a
one-semester work experience program.
We would then measure all of the students
grades prior to the start of the program and then
again after the program.
Those students who participated would be our
treatment group; those who did not would be our
Tim series designs refer to the pretesting
and posttesting of one group of subjects at
The purpose might be to determine long term
effect of treatment and therefore the number of pre-
and posttests can vary from one each to many.
Sometimes there is an interruption between
tests in order to assess the strength of treatment
over an extended time period.
When such a design is employed, the posttest
is referred to as follow-up.
This design is used when we want to compare
two groups that are likely to be different even
before the study begins.
In other words, if we want to see how a new
treatment affects people with different
psychological disorders, the disorders themselves
would create two or more nonequivalent groups.
Once again, the number of pretests and
posttests can vary from one each to many.
obvious concern with all of the quasi-experimental
designs results from the method of choosing subjects
to participate in the experiment.
While we could compare grades and determine
if there was a difference between the two groups
before and after the study, we could not state that
this difference is related to the work experience
itself or some other confounding variable.
It is certainly possible that those who
volunteered for the study were inherently different
in terms of motivation from those who did not
subjects are chosen for groups based on convenience
rather than randomization, the reason for inclusion
in the study itself confounds our results.
5.2: Diagrams of Quasi Experimental Designs