Microsoft Office Tutorials and References
In Depth Information
Now, you need to find the lower limit and the upper limit of the 95% confidence
interval for this study.
We will use Excel’s TINV function to do this. We will assume that you want to
be 95% confident of your results.
Note that this TINV formula uses 24 since 24 is one less than the sample size of
25 (i.e., 24 is n-1). Note that D10 is the mean, while D16 is the standard error of the
mean. The above formula gives the lower limit of the confidence interval, 26.92 .
F23: D10 þ TINV(1-.95,24)*D16
The above formula gives the upper limit of the confidence interval, 29.42.
Now, use number format (two decimal places) in your Excel spreadsheet for the
mean, standard deviation, standard error of the mean, and for both the lower limit
and the upper limit of your confidence interval. If you printed this spreadsheet now,
the lower limit of the confidence interval (26.92) and the upper limit of the
confidence interval (29.42) would “dribble over” onto a second printed page
because the information on the spreadsheet is too large to fit onto one page in its
present format.
So, you need to use Excel’s “Scale to Fit” commands that we discussed in
Chapter 2 (see Sect. 2.4) to reduce the size of the spreadsheet to 95% of its current
size using the Page Layout/Scale to Fit function. Do that now, and notice that the
dotted line to the right of 26.92 and 29.42 indicates that these numbers would now
fit onto one page when the spreadsheet is printed out (see Fig. 3.4 )
Note that you have drawn a picture of the 95% confidence interval beneath cell
B26, including the lower limit, the upper limit, the mean, and the reference value of
28 mpg given in the claim that the company wants to make about the car’s miles per
gallon performance.
Now, let’s write the conclusion to your research study on your spreadsheet:
Since the reference value of 28 is inside
the confidence interval, we accept that
the Chevy Impala does get 28 mpg.
Important note: You are probably wondering why we wrote the conclusion on
three separate lines of the spreadsheet instead of writing it on one long line. This is
because if you wrote it on one line, two things would happen that you would not
like: (1) If you printed the conclusion by reducing the size of the layout of the page
so that the entire spreadsheet would fit onto one page, the print font size for the
entire spreadsheet would be so small that you could not read it without a
magnifying glass, and (2) If you printed the spreadsheet without reducing the
page size layout, it would “dribble over” part of the conclusion to a separate
page all by itself, and your spreadsheet would not look professional.
Your research study accepted the claim that the Chevy Impala did get 28 miles
per gallon on the highway. The average miles per gallon in your study was 28.17.
(See Fig. 3.5 )
Save your resulting spreadsheet as: CHEVY7
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