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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Standard Deviation
The standard deviation method is not really appropriate for this set of data, because the values are not distributed “normally”—that is, in a bell-shaped curve. But just because a method is inappropriate doesn’t mean we can’t have the software apply it. It just means that we get bad results. (Garbage in, garbage out, as the saying goes.)
____ 16. Select the Standard Deviation method, and make the Interval Size one half of a standard deviation. Find and click boxes to show both the Standard Deviation and the Mean on the histogram (with vertical dashed lines). Click OK. Notice that the color ramp changes so that the categories reflect a change at the mean (-0.25 to +0.25), rather than gradual change from one end to the other. Click OK, then Apply, then OK. Observe. You might find the display in the T/C confusing. That’s because the “–” sign is used both as a minus and as a separator.

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