This is the number of businesses with 50 or more employees per 1000 persons by zip code. Is is taken from the U.S. 2006 Economic Census Zip Code Business Patterns and compared against population counts from the 2000 decennial census. It was decided that this would be a more robust number than trying to resolve estimated population projections for 2006 down to the zip code level.
The number of businesses with 50 or more employees in a location as compared to that location's population is a measure of a location's economic base and employment capacity. Over time it is also used to analyze changes in the structure and location of area businesses, and the need to attract new business or retain existing firms.
The economic data is pulled from the U.S. Annual Economic Census ZIP Code Business Patterns (ZCBP) reports for 2006. The population data is from the 2000 census, field P001001: Total Population. They can both be found on FactFinder: