Statistics Dictionary
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Completely Randomized Design
A completely randomized design is probably the simplest
experimental design,
in terms of data analysis and
convenience. With this design, subjects are randomly assigned
to treatments.
| Treatment |
| Placebo |
Vaccine |
| 500 |
500 |
A completely randomized design layout for a hypothetical medical experiment is
shown in the table to the right. In this design, the experimenter
randomly assigned subjects to one of two treatment conditions.
They received a
placebo
or they received a cold vaccine. The same number of subjects (500) are
assigned to each treatment condition (although this is not required).
The dependent variable is the number of colds reported in each
treatment condition. If the vaccine is effective, subjects in
the "vaccine" condition should report significantly fewer colds
than subjects in the "placebo" condition.
A completely randomized design relies on
randomization
to control for the effects of extraneous variables. The experimenter
assumes that, on averge, extraneous factors will affect treatment
conditions equally; so any significant differences between conditions
can fairly be attributed to the independent variable.
