Rate (binomial) Areal Interpolation assumes each member of a population either has or does not have a particular characteristic (such as a particular disease). For each polygon, the population field specifies the total size of the population, and the count field specifies the number of the population members that have the characteristic. Thus, for each polygon, the count cannot be larger than the population. For proportions to be meaningful, it is required that the population be greater than zero.
Identify if there are any problematic values in the attribute table that need to be resolved. Two basic procedures to follow are described below.
- In the Attribute Table of the polygon feature class, sort both the count and population fields to make sure that neither contain any negative values (and that population is not zero).
- Add a numeric field to the attribute table and subtract the count field from the population field, then sort the new field from smallest to largest. Any negative values in the new field indicate that the count field is bigger than the population field, and these incorrect records should be corrected or deleted.