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Chapter 3
Section 3.1
Exercise 3.3
One study of cell phones and the risk of brain cancer looked at a group of 469 people
who have brain cancer. The investigators matched each cancer patient with a person of
the same sex, age, and race who did not have brain cancer, then asked about use of cell
phones. Result: “Our data suggest that use of handheld cellular telephones is not
associated with risk of brain cancer.” Is this an observation study or an experiment?
Why? What are the explanatory and response variables?
From Journal of American Medical Association (2000)
Exercise 3.4
A typical hour of prime-time television shows three to five violent acts. Linking family
interviews and police records shows a clear association between time spent watching TV
as a child and later aggressive behavior. Is this an experiment or observational study?
Why? What are the explanatory and response variables?
Suggest several lurking variables describing a child’s home life that may be confounded
with how much TV he or she watches.
From Science (2002)
Exercise 3.5
An educational software company wants to compare the effectiveness of its computer
animation for teaching cell biology with that of a textbook presentation. The company
tests the biological knowledge of each subject in a group of first-year college students,
then divides them into two groups. One group uses the animation, and the other studies
the text. The company retests all the students and compares the increase in understanding
of cell biology in the two groups. Is this an experiment or an observational study? Why?
What are the explanatory and response variables?
Treatments for more than one explanatory variables
A drug manufacturer is studying how a new drug behaves in patients. Investigators
compare two doses: 5 milligrams (mg) and 10 mg. The drug can be administered by
injection, by a skin patch, or by intravenous drip. Concentration in the blood after 30
minutes (the response variable) may depend both on the dose and on the method of
administration. How many treatments are present in this experiment?
Placebo effect
“Gastric freezing” is a clever treatment for ulcers in the upper intestine. The patient
swallows a deflated balloon with tubes attached, then a refrigerated liquid is pumped
through the balloon for an hour. The idea is that cooling the stomach will reduce its
production of acid and so relieve ulcers. An experiment reported in the Journal of
American Medical Association showed that gastric freezing did reduce acid production
and relieve ulcer pain. The treatment was safe and easy and was widely used for several
years. However, the experiment was poorly designed. The patients’ response may have
been due to the placebo effect. A later experiment divided ulcer patients into two groups.
One group was treated by gastric freezing as before. The other group received a placebo
treatment in which the liquid in the balloon was at body temperature rather than freezing.
The results: 34% of the 82 patients in the treatment group improved, but so did 38% of
the 78 patients in the placebo group. Gastric freezing was then abandoned.