# SOLUTION: 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, st

Algebra ->  Algebra  -> Probability-and-statistics -> SOLUTION: 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, st      Log On

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 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 \$85Answer by stanbon(57387)   (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. ------------- Your sample size is "n". The number whose order is > \$85. The sample proportion is x/n ------------------------------- Find the z-value associated with a 90% two-tail: z = 1.645 -------------------------------- Find the "standard error" = E = 1.645*sigma/sqrt(n) --------------------------------- The 90% CI is (p-hat - E , p-hat+E) ================================================================== and Find the 95% Confidence Interval for the proportion of all customers whose order contains at least 3 items and cost at least \$85 --- Same as above but use the z-value associated with a 95% two-tail: z = 1.96 ============================================================================= Cheers, Stan H>