Question 224172: I am looking for some general help in understanding this concept concerning confidence intervals. I have a file where I have used the data in the spread sheet to find the mean, standard deviation, and some other point estimates. Now I am trying to determine the next step to find the answers to the following questions:
Find the 90% CI for the portion of all customers whose order is more than $85 (since this is a population portion problem I chose to count the number of people whose order is over $85/number in sample to determine phat) Am I on the right track? I want to do something similar for the other part of the question.
and
Find the 95% Confidence Interval for the portion of all customers whose order contains at least 3 items and cost at least $85
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! Find the 90% CI for the portion of all customers whose order is more than $85 (since this is a population portion problem I chose to count the number of people whose order is over $85/number in sample to determine phat) Am I on the right track? I want to do something similar for the other part of the question.
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Your sample size is "n". The number whose order is > $85.
The sample proportion is x/n
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Find the z-value associated with a 90% two-tail: z = 1.645
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Find the "standard error" = E = 1.645*sigma/sqrt(n)
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The 90% CI is (p-hat - E , p-hat+E)
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and
Find the 95% Confidence Interval for the proportion of all customers whose order contains at least 3 items and cost at least $85
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Same as above but use the z-value associated with a 95% two-tail: z = 1.96
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Cheers,
Stan H>
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