Objective: Identify and use the four principles of

Objective: Use experimental methods to design our own
Homework: Read section 6.1 and 6.2 (for
vocab and examples) and then complete
exercises: 6.19, 6.20, 6.21, 6.26, 6.27,
6.28, 6.33, 6.36
What is wrong with this experiment
• A credit card bank wanted to test the
sensitivity of the market to two factors: the
annual fee charged for a card and the
annual percentage rate charged.
• The bank selected 100,000 people at
random from a mailing list. It sent out
50000 offers with a low rate and no fee
and 50000 offers with a higher rate and
a$50 annual fee.
The subjects signed up for the lower rate, no fee
card at twice the rate of the other
• The question the bank wanted to answer
was: how much of the change was due to
the rate and how much was due to the
• How could they have designed their
experiment better?
Lurking / Confounding?
• Lurking variables can be thought of as a
prior cause of both y and x that makes it
appear that x may be causing y ( in
observational studies and in regression).
• A confounding variable is associated in a
noncausal way with a factor and effects
the response. We can’t tell whether the
effect was caused by our factor or by the
confounding variable ( in experiments).
What is it?
• There is found to be a positive association
between ice cream sales and number of
• This is a clear example of a lurking
variable- the season (summer time) has
both more ice cream sales and also more
• A recent medical study found that among men
who have had a heart attack, those with a higher
level of a certain protein in their blood are at
greater risk to have a second attack.
• What can you conclude from this study?
• Nothing much. Maybe the protein caused the
second heart attack, maybe it was just
something that occurred in both, or as a result of
the heart attack.
• A retrospective study that finds most
people with lung cancer were smokers
• A prospective study which follows a group
of smokers and nonsmokers for several
• An experiment on 10 year olds…
From Newsweek:
• Of all the pre-college curricula, the highest level
of mathematics one studies in secondary school
has the strongest continuing influence on
bachelor’s degree completion. Finishing a
course beyond Algebra 2 more than doubles the
odds that a student who enters post-secondary
education will complete a bachelor’s degree.
• Propose a study design to study that might
enable you to draw this conclusion (at least a
causal one)
Ethical Concerns
• Is it okay to use animals for experiments?
Does it matter for what purpose?
• What is “informed consent?”
• Is it okay to drill placebo holes in patients’
skulls so that they and their doctors won’t
know whether or not they received an
experimental implantation of nerve cells as
a possible treatment for Parkinson’s
Disease? (this was actually done).

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