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Hello professor, I was doing the new review questions for chapter seven #75 and the result I got for the population proportion interval were 67.12 % and 76.48 %. Your answer says the results are 74.8% and 83.2%, because my Z score for .02/2 = 2.326 my p^= .718 and q^=.282 and my E=.046807. Can you please tell me where is the mistake? I've done the problem 3 times already. Thank you.

Posted to STATS 2 on Monday, September 22, 2014   Replies: 1

Professor Mcguckian
09/22/2014
7:46 PM EST

Hi Eduardo,

In this problem, it says:

A random sample of 500 students were asked about the costs of traditional textbooks. Three hundred ninety-five of the students stated that traditional textbooks are too expensive. Using the data from the sample, construct a 98% interval estimate of the true proportion of students who believe traditional textbooks are too expensive.

Your sample proportion should be x/n. Here x represents the number of students who think textbooks are too expensive. This will give us 395/500 = 0.79 for p-hat. The q-hat is simply: 1 - 0.79 = 0.21.

That is most likely the source of your error because the rest of your work seems correct. In other words, using your incorrect p-hat and q-hat, I found the answer you found. If you use the correct p-hat and q-hat, you will get the right answer because everything else you did is correct.

Hope that helps,

Professor McGuckian