Designing Studies 4.1 Sampling and Surveys

Just for laughs!

Basic vocabulary Observational study: observes individuals and records data, but does not influence responses Experiment: imposes some treatment on individuals to measure responses Population: the entire group of individuals where our interest lies Sample: part of the population where our interest lies Use information from the sample to draw conclusions about the population

Can you answer these? Identify the population and the sample in each of the following. Be Specific. 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 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

How to Sample Badly Convenience Sampling: choosing easiest to reach individuals Voluntary Response Sampling: individuals choose whether or not to respond Bias: systematically favors certain outcomes

describing 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.

What about these. Identify the sampling method used What about these? Identify the sampling method used. Explain how the sampling method could lead to bias. 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 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

Simple Random Samples Simple Random Samples SRS: sample chosen so that each individual has an equal chance of being selected

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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.

Other Good Sampling Methods Stratified Random Sample: divide individuals into similar groups that are important to response then take a SRS in each group Cluster Sampling: divide into groups or clusters then use all individuals in that group or cluster

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 #3-13 odd