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.