What are the
different major types of research designs? We can classify designs into a simple threefold
classification by asking some key questions. First, does the design use random assignment
to groups? [Don't forget that random assignment is not the same thing as random selection of a sample from a population!] If random
assignment is used, we call the design a randomized experiment or true
experiment. If random assignment is not used, then we have to ask a second question:
Does the design use either multiple groups or multiple waves of measurement?
If the answer is yes, we would label it a quasi-experimental design. If no, we
would call it a non-experimental design. This threefold classification is
especially useful for describing the design with respect to internal validity. A
randomized experiment generally is the strongest of the three designs when your interest
is in establishing a cause-effect relationship. A non-experiment is generally the weakest
in this respect. I have to hasten to add here, that I don't mean that a non-experiment is
the weakest of the the three designs overall, but only with respect to internal validity or causal assessment.
In fact, the simplest form of non-experiment is a one-shot survey design that consists of
nothing but a single observation O. This is probably one of the most common forms
of research and, for some research questions -- especially descriptive ones -- is clearly
a strong design. When I say that the non-experiment is the weakest with respect to
internal validity, all I mean is that it isn't a particularly good method for assessing
the cause-effect relationship that you think might exist between a program and its
outcomes.
To illustrate the
different types of designs, consider one of each in design notation. The first design is a
posttest-only randomized experiment. You can tell it's a randomized experiment because it
has an R at the beginning of each line, indicating random assignment. The second design is
a pre-post nonequivalent groups quasi-experiment. We know it's not a randomized experiment
because random assignment wasn't used. And we know it's not a non-experiment because there
are both multiple groups and multiple waves of measurement. That means it must be a
quasi-experiment. We add the label "nonequivalent" because in this design we do
not explicitly control the assignment and the groups may be nonequivalent or not similar
to each other (see nonequivalent group designs). Finally, we
show a posttest-only nonexperimental design. You might use this design if you want to
study the effects of a natural disaster like a flood or tornado and you want to do so by
interviewing survivors. Notice that in this design, you don't have a comparison group
(e.g., interview in a town down the road the road that didn't have the tornado to see what
differences the tornado caused) and you don't have multiple waves of measurement (e.g., a
pre-tornado level of how people in the ravaged town were doing before the disaster). Does
it make sense to do the non-experimental study? Of course! You could gain lots of valuable
information by well-conducted post-disaster interviews. But you may have a hard time
establishing which of the things you observed are due to the disaster rather than to other
factors like the peculiarities of the town or pre-disaster characteristics.
Copyright ©2006, William M.K. Trochim, All Rights Reserved
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Last Revised: 10/20/2006