11/11/2015
condor.depaul.edu/ymendels/223/a7.htm
Assignment #7
Fall, 2015
Each question is worth 10 points.
Problem 1
The total study time for a statistics assignment in a population of college students was approximately Normally distributed with
a mean of 6.49 hours and standard deviation of 1.05 hours. Suppose you plan to take an SRS of size 150 and compute the
average study time.
(a) What is the mean and standard deviation of your sample?
(b) What is the probability of coming up with a sample that shows that they studied 6.0 hours or less?
Problem 2
You want to rent an unfurnished one‐bedroom apartment in Boston next year. The mean monthly rent for a random sample of
10 apartments advertised in the local newspaper is $1024. Assume that the standard deviation is $275. Find the 90%, 95%, and
99% confidence intervals for the mean monthly rent for this category of apartments.
Look at the 95% confidence interval and say whether the following statement is true or false. “This interval describes the price
of 95% of the rents of all the unfurnished one‐bedroom apartments in the Boston area.” Be sure to explain your answer.
Problem 3
A company manager wants to estimate the average length of phone calls in a call center for a tech support company. In a
sample of 80 calls, the mean duration is 211.4 seconds and the sample standard deviation is 184.8 seconds. Describe the
distribution of this sample. Do you believe this to be a reasonably accurate estimate of the population value?
Problem 4
A newly developed smartphone app is designed to retrieve prices and scheduling information for airine flights. Suppose we
want to study how long it takes on average to retrieve this information. We record the processing time for 100 randomly
selected flight searches. The sample statistics are: sample mean = 10.811 seconds and sample standard deviation = 3.222
seconds. Give your answer as a properly formatted confidence interval statement. Choose the most typical value for C.
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