Unit 4: Collecting Data
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Solutions to Practice Questions
1. A number of confounding factors were proposed in the
article. What were they, and why are they confounding factors?
The "treatment" is the race of the driver. The response
is whether or not the drivers were speeding. A confounding
variable, then, will provide an explanation of why speeds might differ
among racial groups. Let's examine the factors mentioned by the
a) glare on windshields: for this to be a confounder, it
must explain why blacks were speeding at different rates.
Presumably, glare would affect both the ability to classify the
drivers' race, but unless this somehow meant that white speeders were
more often mistakenly classified as black, it is hard to see how this
could be a confounder.
2. Do you agree that these confounders are valid?
3. Can you think of other confounders?
4. A claim is made that speeders (however defined) are more
likely to be black than white. Does this necessarily mean that blacks
are more likely than whites to be speeders? Explain.
Not necessarily. You might try to concoct some numbers
that would demonstrate this.
(from Intro Stats by DeVeaux & Velleman)
Science News reported that depressed individuals cry no more often in response to sad situations than nondepressed people. Researchers studied 23 men and 48 women with major depression, and 9 men and 24 women with no depression. They showed the subjects a sad film about a boy whose father has died, noting whether or not the subjects cried. Women cried more than men, but there were no significant differences between the depressed and nondepressed groups.
5. Is this an observational study or an experiment? Why?
This is an observational study. The "treatment" is whether or
not the subjects are depressed, and the response is whether or not an
individual cries. The treatment was clealry not determined by the