Mobility

Aggregation Without Aggravation: Determining Spatial Contiguity and Joining Geographic Areas Using Hashing

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Executive Summary

Grouping smaller neighboring geographic areas into larger regions based on specified criteria was done using two SAS9 programs. The first program creates a list of areas and their neighbors known as a contiguity table. The table is used by the second program to aggregate geographic areas into larger regions. In public health surveillance, aggregation might be necessary to preserve the confidentiality of those diagnosed with illnesses or to show stable rates in areas with sparse population. An example is provided of the merging of towns and cities in New York State until regions are created with a population of at least 20,000. SAS allows this to be done in a systematic and objective way, using an iterative process for area aggregation.

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