Microsoft Office Tutorials and References
In Depth Information
3.1.6 Using Excel’s TINV Function to Find the Confidence
Interval About the Mean
Objective : To use the TINV function in Excel to find the
confidence interval about the mean
When you use Excel, the formulas for finding the confidence interval are:
Þ s
Lower limit
: ¼
X
TINV
ð
1
0
95
n
1
e
: ð
no spaces between these symbols
Þ
(3.3)
:
;
:
1 Þ s
Upper limit
: ¼
X
þ
TINV
ð 1 0 : 95 ;
n
e
: ð no spaces between these symbols Þ
(3.4)
:
Note that the “* symbol ” in this formula tells Excel to use the multiplication step
in the formula, and it stands for “times” in the way we talk about multiplication.
You will recall from Chapter 1 that n stands for the sample size, and so n
1
stands for the sample size minus one.
You will also recall from Chapter 1 that the standard error of the mean, s.e.,
equals the STDEV divided by the square root of the sample size, n (See Section 1.3 ).
Let’s try a sample problem using Excel to find the 95% confidence interval about
the mean for a problem.
Let’s suppose that General Motors wanted to claim that its Chevy Impala
achieves 28 miles per gallon (mpg) on the highway. Let’s call 28 mpg the “refer-
ence value” for this car.
Suppose that you work for Ford Motor Co. and that you want to check this claim
to see is it holds up based on some research evidence. You decide to collect some
data and to use a two-side 95% confidence interval about the mean to test your
results:
3.1.7 Using Excel to find the 95 Percent Confidence Interval for a
Car’s mpg Claim
Objective : To analyze the data using a two-side 95% confidence
interval about the mean
You select a sample of new car owners for this car and they agree to keep track
of their mileage for two tanks of gas and to record the average miles per gallon they
achieve on these two tanks of gas. Your research study produces the results given in
Fig. 3.1 :
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