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Review and Examples
•
When you know the population standard deviation
σ
, you can use this
conﬁdence interval for the population mean
μ
:
¯
x
±
z
α/
2
σ
√
n
•
When you do not know the population standard deviation
σ
, you ﬁrst
estimate
s
(the sample standard deviation), and then compute the con
ﬁdence interval for
μ
using
¯
x
±
t
α/
2
s
√
n
Example (Creating Conﬁdence intervals for a population mean in Excel)
: The
ﬁle Alcohol.xls contains the annual consumption in liters for a random sample
of 50 European women.
•
Use Excel to compute the sample mean. (Use =AVERAGE(A2:A51) )
•
Use Excel to compute the standard deviation (Use =STDEV(A2:A51) )
•
To compute the conﬁdence interval (note that we do not know
σ
, but
we do know
s
):
1. Click on “Data”
→
“Data Analysis”.
2. Select Descriptive Statistics.
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 Fall '10
 Jager,AbigailL
 Standard Deviation

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