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Randomized Block Design
With a randomized block design, the experimenter
divides subjects into subgroups called blocks,
such that the variability within blocks is less than the
variability between blocks. Then, subjects within each block are
randomly assigned to treatment conditions. Compared to a
completely randomized design,
reduces variability within treatment conditions and potential confounding,
producing a better estimate of treatment effects.
The table to the right shows a randomized block design for a hypothetical
medical experiment. Subjects are assigned to blocks, based on
gender. Then, within each block, subjects are randomly assigned
to treatments (either a
or a cold vaccine). For this design,
250 men get the placebo, 250 men
get the vaccine, 250 women get the placebo, and 250 women
get the vaccine.
It is known that men and women are physiologically different
and react differently to medication. This design ensures that
each treatment condition has an equal proportion of men and women.
As a result, differences between treatment conditions cannot
be attributed to gender. This randomized block design removes
gender as a potential source of variability and as a
potential confounding variable.