AP Statistics
Double Blind Experiments
 Neither the subjects nor the people who have contact
with them know which treatment a subject received.
Matched pairs designs
 Matched pairs designs compare two treatments ONLY.
We choose blocks of two units that are as closely matched
as possible. We assign one of the treatments to each unit
by tossing a coin or reading odd and even digits from Table
B. ALTERNATIVELY, each block in a matched pairs design
may consist of just one subject who gets both treatments
one after the other. Each subject serves as his or her own
control. The order of the treatments can influence the
subject’s response, so we randomize the order for each
subject by a coin toss.
 YOU MUST HAVE INDEPENDENT OBSERVATIONS FOR
A MATCHED PAIRS DESIGN
Example 5.16 p 301
Are cereal leaf beetles more strongly attracted by the color yellow or by the color green? Agriculture researchers
want to know, because they detect the presence of the pests in farm fields by mounting sticky boards to trap insects
that land on them. The board color should attract beetles as strongly as possible. We must design an experiment to
compare yellow and green by mounting boards on poles in a large field of oats.
 WE can do this with a matched pairs design. The
experimental units are locations in the field far enough apart
to represent independent observations.
 We erect a pole at each location to hold the boards.
 We will mount a board of each color on both poles. We will
have to randomly select the color that goes on top and on the
bottom by a coin toss or using odd digits in table B.
 We compare the number of trapped beetles on the green
board with the number of trapped beetles on the yellow
board on the SAME POLE!
A Block Design
 a group of experimental units or subjects that are
known before the experiment to be similar in some
way that is expected to affect the response to the
treatments. In a block design, the random assignment
of units to treatments is carried out separately within
each block.
Example 5.17 Comparing cancer therapies
The progress of a type of cancer differs in women and in men. A clinical experiment to compare three therapies
for this cancer therefore treats sex as a blocking variable. Two separate randomizations are done. One assigning
the female subjects and the other assigning the male subjects. They are groups of subjects that differ in some
way (sex in this case) that is apparent BEFORE the experiment.
 Here is a mapping of this experiment: