defined as a unique location. Whereas a firm could own multiple establishments, all of which can be represented by the headquarters of the firm, establishment data shows locations of all individual establishments in the firm, making establishment data more attractive for studying location choices. The individual business establishments in the data are given as geographically referenced points that include certain establishment attributes and a unique business category code. Geographic coordinates, as well as an address field associated with each establishment in the database, allowed us to match each business to a particular building in the two cities. A business category code associated each establishment with a six digit North American Industry Classification System (NAICS). NAICS is the standard used by federal statistical agencies for classifying business establishments for the purpose of collecting, analyzing, and publishing statistical data related to the U.S. business economy (Census 2009) . The number of NAICS category digits indicates the level of detail in the establishment description 4 . While the two-digit code “44” refers to the highest-level description, called “retail trade”, a three-digit code can distinguish “441” — “Motor Vehicle and Parts Dealers”— from “442”— “Furniture and Home Furnishings Stores”. We also obtained employment data from the InfoUSA database, which listed the estimated number of employees in every establishment. Public transit data for Cambridge and Somerville were obtained from the Massachusetts Bay Transportation Authority (MBTA). The data show the routes and stops of subway and bus lines in both cities. The spatial distribution of residents was obtained from the year 2,000 census records at the block level. Since census data was not originally obtained at the individual building level, it needed to be transformed to a building-level resolution in order to maintain buildings as common units of analysis. To match census counts from blocks to buildings, we first used the assessors databases of Cambridge and Somerville to determine which parcels contained residential buildings and then allocated the total population of each census block between the residential buildings in that block, weighing the allocations by building volume. Large apartment buildings thus obtained a proportionately larger share of residents than small single-family homes 5 . The urban form characteristics were obtained from the MassGIS, as well as the cities of Cambridge and Somerville. The roads data, which we used for all access calculations, are the official state-maintained street transportation dataset available from MassGIS (last updated in December 2007). The roads data represent the centerlines of all local and major streets and roads in the Boston metropolitan area. They also describe the attributes of each road segment in the area, including paved width, sidewalk width, and rightof-way. These road characteristics were spatially joined with individual buildings, so that each building obtained attributes describing the street segment in front of it. Building footprints and heights were obtained from the 2002 LIDAR 6 scan database of MassGIS. Combining building footprints and heights allowed us to estimate each building’s volume in cubic feet. 4
NAICS digits in the InfoUSA database are shown down to the six-digit level, though we only focus on the two- and three-digit categories. 5 We acknowledge that this allocation procedure can introduce additional measurement error to the data. The procedure was necessary, however, for obtaining a common unit of analysis. 6 LIDAR (Light Detection And Ranging) is an optical remote sensing technology that measures the topography of natural and man-made objects on the earth’s surface from an airborne scanner.
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