Introductory Statistics Third Edition
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1 of 3
Menso is an organisation for people with high Intelligence Quotients (IQs).
Menso is investigating the average IQ of primary school students to determine
whether its entry requirements should be altered for younger people. A sample
of 40 primary school students have been randomly selected from schools
throughout the country. The sample mean IQ of those students was calculated
as 107. It is known that the population standard deviation of the IQs of all
people is 10. It is assumed that this standard deviation will also apply
specifically to the IQs of the primary school students.
Calculate the upper and lower bounds of the 99% confidence interval for the mean IQ of primary school
students. You may find this standard normal table useful. Give your answers to 2 decimal places.
a) Upper bound =
b) Lower bound =
2 of 3
As a stock analyst, your boss, Jerry, has asked you to compile some information on stock of Southern
Infrastructure Corporation including a 95% confidence interval for the mean daily return that he needs to
include in a report to senior management. He says that he is also not sure exactly what a 95% confidence
interval means and would like you to add an explanation.
You have been following the share price of Southern Infrastructure Corporation and have recorded the daily
return (as a percentage) for the last 120 days. The data is presented here:
Download the data
Daily returns (%)
Historically, the standard deviation in daily return for this stock is 0.7%.
Complete the report to your boss. Give your numeric answers to 3 decimal places.
May 4, 2016 11:10 AM
Subject: Southern Infrastructure Corp. stock info
Here are the results gathered from the collected data:
Assuming a population standard deviation in daily return of 0.7%, the 95% confidence interval for the mean
daily return is:
b) This means that
approximately 95% of sample means will be within the interval given above
using a process that gives correct results in 95% of cases, the population mean daily return is within
the interval given above
the population mean daily return is definitely within the interval given above
on approximately 95% of days in a given period the stock makes a return within the interval given
3 of 3
WMB is a car manufacturer. The company is trying to estimate the cost of building a
particular car part. Specifically, the company would like to construct a 90%
confidence interval for the mean cost of building this part. For the purposes of
constructing the interval, the company is assuming that the population standard
deviation in this cost is $20.53.
You were dozing in the meeting and so you didn't hear the proposed value for the
margin of error for the confidence interval. However you did hear that, in order to
achieve this proposed margin of error, a sample size of 219 items is required.
You also remember hearing the boss say that the company couldn't afford to collect a sample this large. To
reduce the sample size, a quick decision was made to double the size of the margin of error. All other
parameters of the situation are to remain the same.
Based on this information, calculate the required sample size to construct a confidence interval with this wider
margin of error. Round your answer up to the nearest whole number.
Required sample size =
Perdisco / latin /, v., to learn thoroughly
? 2010 Perdisco
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