Chapter 11: Q.11.132 (page 478)
Obtain a formula for the margin of error, , in estimating the difference between two population proportions by referring to Step 2 of Procedure on page 472 .
Short Answer
The formula of margin of error is
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Chapter 11: Q.11.132 (page 478)
Obtain a formula for the margin of error, , in estimating the difference between two population proportions by referring to Step 2 of Procedure on page 472 .
The formula of margin of error is
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We have given a likely range for the observed value of a sample proportion
a. Based on the given range, identify the educated guess that should be used for the observed value of to calculate the required sample size for a prescribed confidence level and margin of error.
b. Identify the observed values of the sample proportion that will yield a larger margin of error than the one specified if the educated guess is used for the sample-size computation.
In discussing the sample size required for obtaining a confidence interval with a prescribed confidence level and margin of error, we made the following statement: "... we should be aware that, if the observed value of is closer to than is our educated guess, the margin of error will be larger than desired." Explain why.
One-Proportion Plus-Four z-Interval Procedure. To obtain a plus four -interval for a population proportion, we first add two successes and two failures to our data (hence, the term "plus four") and then apply Procedure on page to the new data. In other words, in place of (which is ), we use . Consequently, for a confidence level of , the endpoints of the plus-four -interval are
As a rule of thumb, the one-proportion plus-four -interval procedure should be used only with confidence levels of or greater and sample sizes of or more.
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