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# Random Selection & Assignment

**Random selection** is how you draw the sample of people for
your study from a population.

**Random**is how you assign the sample that you draw to different groups or treatments in your study.

*assignment*It is possible to have *both* random selection and assignment in a
study. Let's say you drew a random sample of 100 clients from a population list of 1000
current clients of your organization. That is random sampling. Now, let's say you randomly
assign 50 of these clients to get some new additional treatment and the other 50 to be
controls. That's random assignment.

It is also possible to have *only one of these* (random selection or random
assignment) but not the other in a study. For instance, if you do not randomly draw the
100 cases from your list of 1000 but instead just take the first 100 on the list, you do
not have random selection. But you could still randomly assign this nonrandom sample to
treatment versus control. Or, you could randomly select 100 from your list of 1000 and
then nonrandomly (haphazardly) assign them to treatment or control.

And, it's possible to have *neither* random selection nor random assignment. In
a typical nonequivalent groups design in education you might nonrandomly choose two 5th
grade classes to be in your study. This is nonrandom selection. Then, you could
arbitrarily assign one to get the new educational program and the other to be the control.
This is nonrandom (or nonequivalent) assignment.

Random selection is related to sampling. Therefore it is most related to the external validity (or generalizability) of your results. After all, we would randomly sample so that our research participants better represent the larger group from which they're drawn. Random assignment is most related to design. In fact, when we randomly assign participants to treatments we have, by definition, an experimental design. Therefore, random assignment is most related to internal validity. After all, we randomly assign in order to help assure that our treatment groups are similar to each other (i.e., equivalent) prior to the treatment.

Copyright ©2006, William M.K. Trochim, All Rights Reserved

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Last Revised: 10/20/2006