6.2. Experimental Design http://www.ck12.org
from one direction, it is possible that plants closer to the sun would perform better and shade the other plants. So it
would be a good idea to block on sun exposure by creating two blocks, one sunny and one not.
You would randomly assign one plant from each variety to each block. Then within each block, randomly assign the
variety to one of the two positions.
This type of design is calledrandomized block design.
Matched Pairs Design
Amatched pairs designis a type of randomized block design in which there are two treatments to apply. For
example, let’s say we were interested in the effectiveness of two different types of running shoes. We might search
for volunteers among regular runners using the database of registered participants in a local distance run. After
personal interviews, a sample of 50 runners who run a similar distance and pace (average speed) on roadways on
a regular basis is chosen. Because you feel that the weight of the runners will directly affect the life of the shoe,
you decided to block on weight. In a matched pairs design, you could list the weights of all 50 runners in order and
then create 25 matched pairs by grouping the weights two at a time. One runner would be randomly assigned shoe
Aand the other would be given shoeB. After a sufficient length of time, the amount of wear on the shoes would be
compared.
In the previous example, there may be some potential confounding influences. Things such as running style, foot
shape, height, or gender may also cause shoes to wear out too quickly or more slowly. It would be more effective
to compare the wear of each shoe on each runner. This is a special type of matched pairs design in which each
experimental unit becomes their own matched pair. Because the matched pair is in fact two different observations