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.

─

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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