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Ch. 21 The Accuracy of Percentages

Learn how to estimate the percentage of a population based on a sample, estimate the SD of a box using the bootstrap method, and calculate confidence intervals.

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Ch. 21 The Accuracy of Percentages

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  1. Ch. 21 The Accuracy of Percentages • Example 1: Using Example 1 from Monday’s class with a sample size of 400 people. What percentage of the population has a college degree?

  2. We can estimate the population percentage based on the sample pecentage. But how confident are we of our estimate? • Estimate the SD of the box • What is the underlying box model? • Bootstrap method: When sampling from a box of 1’s & 0’s whose composition is unknown, the SD of the box can be estimated by substituting the fractions of 1’s & 0’s in the sampe for the unknown fractions in the box.

  3. This estimate is good as long as the sample is reasonably large. • Confidence intervals • Example 1 (continued): What is the chance that the sample percentage of people with college degrees will fall within the range of the population percentage +/- 2 SE’s? • Margin of error: 2 SE’s is the 95% margin of error.

  4. Activity: Confidence intervals

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