ZIP codes are used by the US Postal Service to identify an area where mail is delivered. The use of the ZIP code in small area analyses is widespread despite its limitations. The limitations include foremost the non-homogeneity of the ZIP code unit with respect to geographical size and with respect to population size. For instance, California wide in the year 2000, the number of births in ZIP codes ranged from 0 to 2745 with a mean of 246 births and a standard deviation of 368 births.

The obvious advantage of using the ZIP code as the unit of analysis is its wide availability: ZIP code information can be easily collected compared to information on census block group. The wide availability of ZIP code level data allows the combining of ZIP code level information from different databases.

The table below lists all ZIP codes for Colusa County that were included in the most recent list of available ZIP codes. The listing as well as the map coordinate files for the maps included in this report were based on data products that were obtained from the TeleAtlas company.

Among the USPS ZIP codes, not all have a physical service area. Point ZIP codes pertaining to a unique ZIP code (ZIP codes pertaining to a single site, building, organization) or P.O. Box cannot be associated with a physical area on a map. For the purposes of this report any counts in point ZIP are included in the counts for the enclosing ZIP. For each point ZIP, the enclosing ZIP is listed in the table below.

ZIP Inventory Table

ZIP Code Inventory for Colusa County, 2007

ZIP Code	Post Office Name	Zip Code Type	Enclosing ZIP Code
95423	Clearlake Oaks	Non-unique ZIP
95912	Arbuckle	Non-unique ZIP
95932	Colusa	Non-unique ZIP
95950	Grimes	P.O. Box	95912
95955	Maxwell	P.O. Box
95970	Princeton	Non-unique ZIP
95979	Stonyford	Non-unique ZIP
95987	Williams	Non-unique ZIP

Source: TeleAtlas

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