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DESIGNING STUDIES
4.1 SAMPLING AND SURVEYS
JUST FOR LAUGHS!
CAN YOU ANSWER THESE?
IDENTIFY THE POPULATION AND THE SAMPLE IN EACH OF
THE FOLLOWING. BE SPECIFIC.
1.
A furniture maker buys hardwood in large batches. The supplier is
supposed to dry the wood before shipping (wood that isn’t dry won’t hold
its size and shape). The furniture maker chooses five pieces of wood from
each batch and tests their moisture content. If any piece exceeds 12%
moisture content, the entire batch is sent back.
• Population: all pieces of hardwood in a batch Sample: 5 pieces of wood selected for
moisture content tests
2.
Each week, the Gallup Poll questions a sample of about 1500 adult U.S.
residents to determine national opinion on a wide variety of issues.
• Population: all adult U.S. residents Sample: 1500 adults responding to the survey
WHAT ABOUT THESE? IDENTIFY THE SAMPLING METHOD
USED. EXPLAIN HOW THE SAMPLING METHOD COULD
LEAD TO BIAS.
1.
A farmer brings a juice company several crates of oranges each week.
A company inspector looks at 10 oranges from the top of each crate
before deciding whether to buy all the oranges.
• Convenience Sampling - overestimate the quality of the oranges best on top
2.
The ABC program Nightline once asked whether the United Nations
should continue to have its headquarters in the United States. Viewers
were invited to call one telephone number to respond “Yes” and another
for “No.” There was a charge for calling either number. More than
186,000 callers responded and 67% said ”No.”
• Voluntary Response Sampling – those opposed more likely to voice their opinion
AP EXAM TIP: IF ASKED TO DESCRIBE HOW THE
DESIGN OF A STUDY LEADS TO BIAS
• Identify a problem with the design
• Explain how the problem would lead to an underestimate or an overestimate
• Example on page 212 in the box.
AP EXAM COMMON ERRORS
• Describing how to select a sample using a random number generator, don’t
forget to address what to do with repeated integers.
• State that repeated integers must be ignored
• Generate random integers until they get n different numbers
• Each label must have the same number of digits
• For 50 labels, use 01-50 not 1-50.
STRATIFIED VS CLUSTER
• The manager of a sports arena wants to learn more about the financial
status of the people who are attending an NBA basketball game. He
would like to give a survey to a representative sample of the more than
20,000 fans in attendance. Ticket prices for the game vary a great deal:
seats near the court cost over $100 each, while seats in the top rows of
the arena cost $25 each. The arena is divided into 30 numbered sections
from 101 to 130. Each section has rows of seats labeled with letters from
A (nearest the court to ZZ (top row of the arena.)
• Explain why it might be difficult to give the survey to an SRS of 200 fans.
• Which would be a better way to take a stratified random sample of fans: using the
lettered rows or the numbered sections as strata? Explain.
• Which would be a better way to take a cluster sample of fans: using the lettered
rows or the numbered sections as clusters? Explain.
4.1 HOMEWORK
•Pg.229 #5-13 odd, 17-35 odd