Mean/Standard Deviation
When you choose Mean/standard deviation as the binning method, a new set of options is displayed in the dialog box.
This method generates one or more new fields with banded categories based on the values of the mean and standard deviation of the distribution of the specified field(s). Select the number of deviations to use below.
Name extension. Specify an extension to use for the generated field(s). _SDBIN is the default extension. You may also specify whether the extension is added to the start (Prefix) or end (Suffix) of the field name. For example, you could generate a new field called income_SDBIN.
- +/– 1 standard deviation. Select to generate three bins.
- +/– 2 standard deviations. Select to generate five bins.
- +/– 3 standard deviations. Select to generate seven bins.
For example, selecting +/–1 standard deviation results in the three bins as calculated and shown in the following table.
| Bin 1 | Bin 2 | Bin 3 |
|---|---|---|
x < (Mean – Std. Dev)
|
(Mean – Std. Dev) <= x <= (Mean + Std. Dev)
|
x > (Mean + Std. Dev)
|
In a normal distribution, 68% of the cases fall within one standard deviation of the mean, 95% within two standard deviations, and 99% within three standard deviations. Note, however, that creating banded categories based on standard deviations may result in some bins being defined outside the actual data range and even outside the range of possible data values (for example, a negative salary range).