Microsoft Office Tutorials and References
In Depth Information
Deviating from the Middle
Figure 9-8:
Normal
distributions come
in different
heights and
widths.
All normal distributions have a mean and a standard deviation. However, the
standard normal distribution is characterized by having the mean equal 0 and
the standard deviation equal 1.
A table of values serves as a lookup in determining probabilities for areas
under the standard normal curve. This table is useful when working with data
that has been modified to fit the standard normal distribution. This table is
often found in the appendix section of statistics books and on the Internet as
well. Try www.statsoft.com/textbook/sttable.html.
Using STANDARDIZE
In order to use this table of standard normal curve probabilities, the data
being analyzed must be standardized. Excel provides the STANDARDIZE
function for just this purpose. STANDARDIZE takes three arguments: the data
point, the mean, and the standard deviation. The returned value is what the
data point value is when the mean is 0 and the standard deviation is 1.
An individual value from a nonstandard normal distribution is referred to as x.
An individual value from a standard normal distribution is referred to as z.
Figure 9-9 shows how the STANDARDIZE function changes raw values to
standard values. The standard deviation of the raw data is 7.438637452, but the
standard deviation of the standardized values is 1. The mean of the
standardized values is 0.
Column B in Figure 9-9 has a long list of 1,200 random values. The mean is
17.23473829, as seen in cell C2. The standard deviation is 7.438637452, as
seen in cell C3. For each data point in Column B, the standardized value is
displayed in Column E. The list of values in Column E are those returned with
the STANDARDIZE function.
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