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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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Quantiles
The problem of empty classes may be solved by the approach of quantiles. Quantiles places approximately the same number of values in each category.
____ 15. Classify and select the Quantile method. Change the number of classes back to 5 (this gives you quintiles; other popular classifications are quartiles, deciles, and percentiles). Click OK, Apply, and OK. Notice the map and the Table of Contents. Again, the pattern looks somewhat similar to previous ones, but here the values around 1000 are broken into two groups. A further complication: two of the values around 5000 are lumped in with some 1000s, while the other two are in with the 10000s and the 15000s. Again, the divisions seem inappropriate.

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