Samples, Good and Bad

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Chapter 2

• Samples, Good and Bad

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Thought Question 1
Advice columnist Ann Landers often asked readers
to write and tell her their feelings about
certain topics, like whether or not they think
engineers make good husbands. Do you think the
responses she received were representative of
public opinion? Explain why or why not.
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Thought Question 2
Taste tests of new products are often done by
having people taste both the new product and an
old familiar standard. Do you think the results
would be biased if the person handing the
products to the respondents knew which was which?
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Thought Question 3
A newsletter distributed by a politician to his
constituents gave the results of a nationwide
survey on Americans attitudes about a variety of
educational issues. One of the questions asked
was, Should your legislature adopt a policy to
assist children in failing schools to opt out of
that school and attend an alternative
school--public, private, or parochial--of the
parents choosing? From the wording of this
question, can you speculate on what answer was
desired? Explain.
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Thought Question 4
Many television stations conduct polls by asking
viewers to call one phone number if they feel one
way about an issue and a different phone number
if they feel the opposite. Do you think the
results of such a poll represent the feelings of
the community? Do you think they represent the
feelings of all those watching the TV station at
the time or of some other group? Explain.
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Thought Question 5
Suppose you had a telephone directory listing all
of the businesses in a city, alphabetized by type
of business. If you wanted to phone 100 of them
in order to get a representative sampling of
opinion on some issue, how would you select which
100 to phone? Why would it not be a good idea to
simply use the first 100 businesses listed?
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Bad Sampling Plans

• Convenience sampling
• selecting individuals that are easiest to reach
• Voluntary response sampling
• allowing individuals to choose to be in the
  sample
• Both of these techniques are biased
• systematically favor certain outcomes

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Convenience Sampling

• Sampling mice from a large cage to study how a
  drug affects physical activity
• lab assistant reaches into the cage to select the
  mice one at a time until 10 are chosen
• Which mice will likely be chosen?
• could this sample yield biased results?

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Voluntary Response

• To prepare for her book Women and Love, Shere
  Hite sent questionnaires to 100,000 women asking
  about love, sex, and relationships.
• 4.5 responded
• Hite used those responses to write her book
• Moore (Statistics Concepts and Controversies,
  1997) noted
• respondents were fed up with men and eager to
  fight them
• the anger became the theme of the book
• but angry women are more likely to respond

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Simple Random Sampling

• Each individual in the population has the same
  chance of being chosen for the sample
• Each group of individuals (in the population) of
  the required size (n) has the same chance of
  being the sample actually selected
• Random selection
• drawing names out of a hat
• random number table (see Table A on pg. 545 of
  text, or Random Number Table on pg. 178 of this
  supplement)
• computer software

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Key Concepts

• Bias
• Convenience Sampling
• Voluntary Response Sampling
• Simple Random Sampling