David B. Howell
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A. Objectives—students will be able to
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1-5. same as Lesson 4.
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6. select a sample size to estimate a population characteristic with confidence.
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B. The experimental question—What percent of the population read “Ebony” in the last week?
C. Issues, and some possible resolutions—
[Let’s assume a student body of 500 to 1500] We have several samples, ranging in size from 20 25 up to combinations of samples totalling 200 or more. Suppose we work for “Ebony,” and it costs us a lot of money to sample each additional group of 20 25. At what sample size would you be willing to stop? When will you say that your sample is large enough for you to predict what percent of the total student body reads “Ebony?” (My answer with 10 seems right. Our class sample of 23 is close. Many groups of 50 seem OK. All the 100 groups are close together—maybe one would have been enough.)
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D. Observations and discussion to Objectives—
Do we need to predict exactly? Is a range of 5 or 10 percent good enough? Does your answer depend on how wide a range is sufficient? (Sure! The single class range is 40%. The 50 group range is only 20%.) Is there a sample size beyond which we don’t seem to be getting any better information? (The 200s don’t look much better than the 100s. Maybe we could stop at 100. What do we need to know for—printing quantities? The prediction from almost all the 50s is good enough.)