[CTPP] CTPP Part 3 Issues - Anniversary Report (long post, with small attachment)

TO: CTPP-News: FR: Chuck Purvis, MTC RE: CTPP Part 3 Issues On the one-year anniversary of receiving the Census 2000 CTPP Part 3 data, I thought I would re-release an old e-mail looking at the Part 3 data suppression and data loss issues (with some current observations). There are 14 total tables in the CTPP Part 3 area-to-area "flow"data. Five of these tables (3-3 through 3-7) have a minimum 3-worker sample threshold applied. If there are less than 3 sample workers, total, on an area-to-area basis, then values for that flow interchange are suppressed. This can have a significant impact at finer geographic summary levels, e.g., tract-to-tract, zone-to-zone, and block group-to-block group. (This is not 'new' news, but is old, bad news.) This particular data analysis is for the Part 3 data for intra-San Francisco Bay Area. Other regions of various sizes will probably show similar patterns. Attached is my "bottom line" spreadsheet that shows the data loss due to suppression and rounding error across all geographic summary levels. Table 3-1 problems are limited solely to rounding errors. There are insignificant rounding errors going from the county-to-county to the place-to-place geographic summary levels, but modest rounding errors at the tract-to-tract level (a loss of 1.5%) and an incredible rounding loss of 3.5 percent at the zone-to-zone level. Note that rounding is done on an interchange cell by interchange cell basis. There is no "bucket rounding" applied in the CTPP to correct for this rounding error. Table 3-2 (household workers by means of transportation) also shows a similar rounding error problem, topping out at a 4.2 percent loss due to rounding at the zone-to-zone level. Table 3-6, all workers by detailed means of transportation, shows minor data loss due to rounding and suppression at the place-to-place level (-1.2 percent), but major data losses at the tract-to-tract level (-34.4 percent), and zone-to-zone level (-71.5 percent). Yes, 71.5 percent of the detailed means of transportation is lost at the zone-to-zone level. The only usable information from Table 3-6, at the zone-level, is the number of at home workers (Table 3-6, cell 18.) (NOTE: If you need block group or tract level workers working AT HOME, USE SUMMARY FILE #3 DATA!!!) The CTPP Part 3 does NOT have an "all workers universe" table on workers by DETAILED means of transportation, WITHOUT the sample size threshold. On the other hand, we can derive "all workers by means of transportation" by dividing the "aggregate travel time by means" (Table 3-14) by the "average travel time by means" (Table 3-8). This "back door" approach to estimating workers by means of transportation (universe: all workers) works exceptionally well. There is basically a negligible data loss across all geographic summary levels. For the past year we have been working with this "derived" commuters, zone-to-zone, based on dividing Table 3-14 by Table 3-8. This is essential if you're examining commuter flows, average commute distances (in miles or kilometers) and commute length frequency distributions. One problem with the Table 3-14 / 3-8 commuters is the "Other" category which combines five sub-groups: bicycle, walk, motorcycle, taxicab, and "other means". What this really means is that we (and probably all others in the U.S.) will be unable to create zone-to-zone bicycle or walk commuter matrices. (This is unlike the 1990 Census, where we created zone-to-zone bicycle and walk commuter matrices for purposes of analyzing commute distances and commute distance frequency distributions for these non-motorized commute modes....) (Actually, the Part 3 data came too late (May 2004) to be used in our travel demand model validation efforts, which we completed April 2004. We used the county-to-county flow data, derived from the 5% PUMS, by means of transportation by household income quartile, for validating our work trip distribution and work trip mode choice models.) That's all I have to offer for the moment. Think it over, and hope to see folks at the Census / Transportation Conference in Irvine, CA this week! Chuck Purvis, MTC ************************************************************** Charles L. Purvis, AICP Principal Transportation Planner/Analyst Metropolitan Transportation Commission 101 Eighth Street Oakland, CA 94607-4700 (510) 464-7731 (office) (510) 464-7848 (fax) www: http://www.mtc.ca.gov/ Census WWW: http://www.bayareacensus.ca.gov/ **************************************************************