Experimental Design
KEY
Name_____________________
Use for Questions 1 to 4: Identify the experimental units or subjects, the factors, the
treatments, and the response variables and then outline the design of the experiment.
1. Ability to grow in shade may help pines found in the dry forests of Arizona to resist
drought. How well do pines grow in shade? Investigators planted pine seedlings in a
greenhouse in either full light, light reduced to 25% of normal by shade cloth, or light
reduced to 5% of normal. At the end of the study, they dried the young trees and
weighed them.
Experimental Units: Pine Seedlings
Explanatory Variable: Light Intensity
Treatments: Full Light, 25% Light, and 5% Light
Response Variable: Dry weight at the end of the study
2. Sickle-cell disease is an inherited disorder of the red blood cells that in the United
States affect mostly blacks. It can cause severe pain and many complications. Can the
drug hydroxyurea reduce the severe pain caused by sickle-cell disease? A study by the
National Institutes of Health gave the drug to 150 sickle-cell sufferers and a placebo to
another 150. The researchers then counted the episodes of pain reported by each
subject.
Experimental Units: 300 sickle-cell patients
Explanatory Variable: Drug Given
Treatments: hydroxyurea and placebo
Response Variable: number of pain episodes
3. Is diet or exercise effective in combating insomnia? Some believe that cutting out
desserts can help alleviate the problem, while others recommend exercise. Forty
volunteers suffering from insomnia agreed to participate in a month-long experiment.
Half were randomly assigned to a special no-desserts diet; the others continued desserts
as usual. Half of the people in each of these groups were randomly assigned to an
exercise program, while the others did not exercise. Those who ate no desserts and
engaged in exercise showed the most improvement.
Experimental Unit: People suffering from insomnia
Explanatory Variable: Dessert and Exercise
Treatments: 4 Treatments Dessert/Exercise, Dessert/No Exercise, No Dessert/Exercise,
Dessert/Exercise
Response Variable: Improvement in ability to sleep
4. Some schools teach reading using phonics (the sounds made by letters) and other using
whole language (word recognition). Suppose a school district want to know which method
works better. They use the PACT scored to determine reading ability.
Experimental Unit: Reading Students
Explanatory Variable: Method Used
Treatments: Phonics and Whole Language (word recognition)
Response Variable: PACT Scores
5. In the mid 1900s, a common treatment for angina (a disease marked by brief attacks of
chest pain caused by insufficient oxygen to the heart) was called internal mammary
ligation. In this procedure doctors made small incisions in the chest and tied knots in two
arteries to try to increase blood flow to the heart. It was a popular procedure – 90% of
patients reported that it helped reduce pain. In 1960, Seattle cardiologist Dr. Leon Cobb
carried out an experiment where he compared ligation with a procedure in which he made
incisions but did not tie off the arteries. This sham operation proved just as successful,
and the ligation procedure was abandoned as a treatment for angina.
a. What is the response variable in Dr. Cobb’s experiment?
The amount of chest pain.
b. Dr. Cobb showed that the sham operation was just as successful as ligation. What
term do we use to describe the phenomenon that many subjects report good results
from a pretend treatment?
A placebo effect.
c. The ligation procedure is an example of the lack of an important property of a welldesigned experiment. What is that property?
There is no control group in this experiment.
6. Doctors identify “chronic tension-type headaches” as headaches that occur almost daily
for at least six months. Can antidepressant medications or stress management training
reduce the number and severity of these headaches? Are both together more effective
than either alone? Investigators compared four treatments: antidepressant alone,
placebo alone, antidepressant plus stress management, and placebo plus stress
management. Outline the design of the experiment. The headache sufferers named in
the following table have agreed to participate in the study. Use a graphing calculator to
randomly assign the subjects to the treatments.
Acosta
Asihiro
Bennett
Bikalis
Chen
Clemente
Duncan
Durr
Edwards
Farouk
Fleming
George
Han
Howard
Hruska
Imrani
James
Kaplan
Liang
Maldonado
Marsden
Montoya
O'Brian
Ogle
Padilla
Plochman
Rosen
Solomon
Trujillo
Tullock
Valasco
Vaughn
Wei
Wilder
Willis
Zhang
a)
This type of design is called a completely randomized design.
b)
RA Using a GDC: Arrange the names alphabetically. Assign each patient a unique number from
01 to 36. Using a GDC and the random integer function, find randint(1,36). The first 9 unique
numbers selected and their corresponding names will be placed in Group 1. The second set of 9
unique numbers/patients will be placed in Group 2. The 3 rd set of 9 unique numbers/patients will
be placed in Group 3. The 4th set of 9 unique numbers/patients will be placed in Group 4.
RA with no GDC:
Write the names of the patients on 36 identical slips of paper, put them in
a hat, and mix them well. Draw out 9 slips without replacement. The corresponding patients will
be in Group 1. Draw out 9 more slips without replacement. Those patients will be in Group 2.
The next 9 slips drawn without replacement will be the patients in Group 3, and the remaining 9
patients will be assigned to Group 4.