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Press the STAT key.
Press the ► twice to highlight TESTS.
Press the number 7 key.
The first line under ZInterval has two options for Inpt: Data Stats. If the mean and standard
deviation are given in the problem then Stats should be selected. If the data values are given in
the problem then Data should be selected. In this problem the mean and standard are given.
Select Stats by moving the cursor over Stats and then press
the ENTER key.
Type in 4.5 for σ.
Type in 70.5 for Ë.
Type in 36 for n.
Type in .90 for C-Level.
Highlight Calculate and then press the Enter key.
The ZInterval output shows the 90% confidence level along
with the sample mean and sample size.
The 90% confidence interval is between $69.27 and $71.34.
This says that with 90% confidence we believe that the true
population mean price is between $69.27 and $71.34.
Unknown Standard Deviation
Most of the time you will not know the population standard deviation. In this case you would use
a T-based confidence interval to estimate the population mean µ. There is one extra condition:
either your population should be normal or your sample size should be larger than 30.
Example: Household Debt
According to a report by the Consumer Federation of America National Credit Union
Foundation, households with negative assets carried an average of $15,528 in debt in the year
2002. Assume that this mean was based on a random sample of 400 households and that the
standard deviation of debts for households in this sample was $4200. Make a 99% confidence
interval for the 2002 mean debt for all such households.
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