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:
Lower limit : ¼ X TINV ð 1 0 : 95 ; n 1 Þ s : e :
ð no spaces between these symbols Þ
ð 3 : 3 Þ
Upper limit : ¼ X þ TINV ð 1 0 : 95 ; n 1 Þ s : e :
ð no spaces between these symbols Þ
ð 3 : 4 Þ
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 Chap. 1 that n stands for the sample size, and so n - 1
stands for the sample size minus one.
You will also recall from Chap. 1 that the standard error of the mean, s.e.,
equals the STDEV divided by the square root of the sample size, n (See Sect. 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). Let’s call 28 mpg the ‘‘reference 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 % 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
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