When you use your own weight set in an interpolation procedure, you will have the option of normalizing the weights. This means that each individual weight value (geographic multiplied by user-assigned weight) is divided by the sum of the weights assigned to all of the units being interpolated. Thus, the sum of all weights will equal one. This option allows you to compute a population-weighted average when applied to rate data weighted by a population dataset, or by other types of count data.

http://www.biomedware.com/files/documentation/spacestat/Statistics/Interpolation/Spatial_Interpolation.htm#Normalize

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