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Area Code NPANXX Database Detailed SpecificationsQuestions? 1-800-425-1169
Detail of Data
Please note: Not every record will have all fields populated
ZIP Code frequency of occurence
ZIP Code frequency of occurrence means the percent of all listed subscribers within an NPA/NXX that have a particular ZIP Code. ZIP Codes that have a frequency of occurrence of less than 2% are omitted from the data. That's because those ZIP Codes are statistically insignificant and are quite likely better represented in an adjacent NPA/NXX.
While ZIP Code boundaries rarely change and CO boundaries essentially never change, there is little deliberate alignment of those boundaries. Consequently, there's nearly always overlap of ZIP Code between adjacent COs (NPA/NXX). By evaluating the frequency of occurrence, you can determine a ZIP Code's relevance within an NPA/NXX relative of it's occurrence in an adjacent NPA/NXX.
When using the data to match ZIP Codes to NPA/NXXs, you'll want to consider both a reasonable frequency of occurrence and actual ZIP Code count to help you determine the accuracy of the match. Selecting reasonable frequency of occurrence and ZIP Code count values are up to you depending on your application, but the higher the value the more accurate the match.
ZIP Code Sourcing
Some NXXs have detailed ZIP Code data (Level 2, detailed) where detailed data were available. Other NXXs may only show one or two ZIP Codes without a frequency of occurrence (Level 1, not as detailed). Here is the key to values in the frequency of occurrence field.
If the frequency of occurrence is -1 or -2, then the ZIP Code count for this record is meaningless -- ignore it.