Hawth's Analysis Tools for ArcGIS

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 Summary

RANDOM SELECTION WITHIN SUBSETS
Input: any vector shapefile / coverage (point, line or polygon)
Output: a random selection of features within each subset (defined by a unique ID field), and optionally a new field called RNDSELSS that marks the selected records with 1
Features:

• for each subset of the data (defined here as a collection of features sharing a unique identifier), the tool will generate a random selection of the features
• the number of features to be selected can be specified as an absolute number, or a percentage of the number of features in that subset
• if the user opts to write a RNDSELSS field to the attribute table, it will overwrite any pre-existing RNDSELSS field and code the selected features with a 1 and the non-selected features with a 0

Limitations:

• the random selection part of this tool should work with geodatabases, but I suspect the option to add a field to the attribute table will not

 Help

Getting started The vector layer must be loaded into ArcMap before starting this tool. This layer must contain a unique ID field that identifies the subsets you wish to sample. For instance, if you are randomly selecting a certain number of polygons of each landcvoer type from a landcover layer, then the unique ID field would be the field containing the landcover codes, not the polygon ID field. layer, the

Creating the random selection Users may specify either a constant number of features to select, or a percentage. If the constant number you supply is equal to or greater than the number of features in any one of the subsets, then all the features in that subset will be selected. For instance, if one of the subsets contains 35 features and you enter 40 as the number of features to select, the resulting selection will contain all 35 of the selected features, and it will not inform you that it was unable to select 40 (so you need to be cognizant of how many features you have).

Writing the RNDSELSS field If you check the option to add or update this field with the new selection then it is important to realize that any previous RNDSELSS field will be overwritten, and that the tool will fail if you do not have write permissions to that attribute table.