1. Chicago Traffic Analysis Zones 9-Counties 1990 Population: 7,429,181 Area (sq. miles): 137 Number of zones: 14,127 People per zone: 526 Resident workers: 3,563,603 Work place workers: 3,635,769 Workers at home: 76,371 Total households: 2,675,257 Counties include: Cook, DuPage, Grundy, Kane, Kankakee, Kendall, Lake, McHenry, and Will Disclosure and Utility of Census Journey-to-Work Flow Data from the American Community Survey Is There a Right Balance? Ed Christopher (Federal Highway Administration) Nanda Srinivasan (Cambridge Systematics, Inc.) Early in 2003 the transportation community contracted with the Census Bureau to produce the CTPP2000, a special tabulation. A special tabulation is made up of user defined tables and falls outside the “standard” products distributed by the Census Bureau like SF1, SF3, and PUMS. With the 2000 decennial data, the Census Bureau required all special tabulations to have disclosure avoidance techniques applied to them. For CTPP2000 this meant the institution of rounding and threshold techniques in addition to the already applied procedures of data swapping and imputation. The specific disclosure rules for the ACS after 5 years of data collection are likely to be similar, if not stricter than to those used for CTPP2000. In this poster the effects of rounding and thresholds on the CTPP are exposed along with an examination of their effects under the ACS. Franklin County (Columbus, OH) Total Workers Living and Working in the County (Census 2000) = 508,393 County-CountyPlace-PlaceTract-TractZone-Zone CTPP2000 Table 3-01 (No Thresholds)508,395508,361500,426487,979 Percent Loss0.00%0.01%1.57%4.02% Table 3-06 (Thresholds)508,395507,604358,170177,643 Percent Loss0.00%0.16%29.55%65.06% Total Workers Living and Working in the County (ACS, 3-yr) = 498,220 ACS (1999, 2000 and 2001) Table 3-01 (No Thresholds)498,220498,168447,446na Percent Loss0.00%0.01%10.19% Table 3-03 (Thresholds)498,220495,840233,920na Percent Loss0.00%0.48%53.05% Douglas County (Omaha, NE) Total Workers Living and Working in the County (Census 2000) = 213,642 County-CountyPlace-PlaceTract-TractZone-Zone CTPP2000 Table 3-01 (No Thresholds)213,640213,655211,565209,315 Percent Loss0.00%-0.01%0.97%2.03% Table 3-06 (Thresholds)213,640 157,334109,247 Percent Loss0.00% 26.36%48.86% Total Workers Living and Working in the County (ACS, 3-yr) = 209,970 ACS (1999, 2000 and 2001) Table 3-01 (No Thresholds)209,970 190,287190,145 Percent Loss0.00% 9.37%9.44% Table 3-03 (Thresholds)209,970209,960124,10379,665 Percent Loss0.00% 40.89%62.06% Pima County (Tucson, AZ) Total Workers Living and Working in the County (Census 2000) = 359,296 County-CountyPlace-PlaceTract-TractZone-Zone CTPP2000 Table 3-01 (No Thresholds)359,295359,281357,695354,566 Percent Loss0.00% 0.45%1.32% Table 3-06 (Thresholds)359,295358,204264,146126,218 Percent Loss0.00%0.30%26.48%64.87% Total Workers Living and Working in the County (ACS, 3-yr) = 354,130 ACS (1999, 2000 and 2001) Table 3-01 (No Thresholds)354,130354,164314,781316,878 Percent Loss0.00%-0.01%11.11%10.52% Table 3-03 (Thresholds)354,130352,635197,92487,319 Percent Loss0.00%0.42%44.11%75.34% County and State Name Most Detailed Geography Percent of Housing Units Sampled Percent of Population Responding CTPPACSCTPPACS Pima AZTAZ12.513.412.78.6 San Fran. CATAZ11.79.611.85.5 Broward FLTAZ11.79.511.55.9 Lake ILTAZ14.310.314.46.6 Hampden MATract13.414.613.59.4 Douglas NETAZ13.915.213.910.5 Bronx NYTract11.310.211.64.4 Franklin OHTract14.19.414.16.2 Multnomah ORTract14.115.014.010.0 Key Data Issues ACSCTPP Rounding RulesSame Group QuartersNoYes Threshold RulesSame Extended AllocationNoYes Housing Units Sampled12.7%13.5% Population Responding8.4%13.6% Los Angeles Census Tracts Boston Census Block Groups 6-Counties 1990 Population:14,640,832 Area (sq. miles): 578 Number of Tracts: 3,934 People per Tract: 3,722 Resident workers: 6,844,948 Work place workers: 6,849,916 Workers at home: 187,091 Total households: 4,942,075 Counties include: Imperial, Los Angeles, Orange, Riverside, San Benardino and Ventura Counties (see below) 1990 Population: 4,056,947 Area (sq. miles): 809 Number of BGs: 3,850 People per BG: 1,054 Resident workers: 2,073,508 Work place workers: 2,201,473 Workers at home: 50,989 Total households: 1,507,077 Counties include: All MCDs in 1990 Boston definition including parts of Middlesex, Essex Worcester, Suffolk, Norfolk, Bristol and Plymouth OD Pairs Lost Due To Thresholds Franklin County (Columbus, OH) CTPP OD Pairs w/Trips ACS OD Pairs w/Trips Without Thresholds With Thresholds Percent Lost Without Thresholds With Thresholds Pairs Lost Place-Place38430620%33422931% Tract-Tract23,2896,79471%13,3802,45982% BG-BG44,2665,04589%----- Douglas County (Omaha, NE) CTPP OD Pairs w/Trips ACS OD Pairs w/Trips Without Thresholds With Thresholds Percent Lost Without Thresholds With Thresholds Pairs Lost Place-Place15147%15147% Tract-Tract8,8303,04466%7,4852,08972% TAZ-TAZ14,3893,08179%11,2691,80984% Pima County (Tucson, AZ) CTPP OD Pairs w/Trips ACS OD Pairs w/Trips Without Thresholds With Thresholds Percent Lost Without Thresholds With Thresholds Pairs Lost Place-Place31820934%27017535% Tract-Tract13,3204,64465%10,5732,91172% TAZ-TAZ26,7813,17988%18,1681,67591% CTPP2000 Disclosure Avoidance Rules Study Areas Used for Rounding Analysis Number of Trips per OD Pair Chicago – TAZ Los Angeles – Tract Boston – BG FrequencyPercent Cum Percent FrequencyPercent FrequencyPercent 11,0750.3 3770.1 1100.1 29,2272.73.01,7270.40.58630.5 36,3721.94.94,2781.01.51,2480.71.2 410,8253.28.013,1613.24.74,7112.63.8 529,2598.516.630,1387.312.013,6967.511.3 647,01613.730.344,77910.822.823,58813.024.3 750,17314.644.946,76411.334.026,46714.538.8 843,48412.757.642,95710.344.424,07313.252.1 933,4599.867.434,9078.452.818,91410.462.5 10+111,92132.6100.0196,09847.2100.068,27837.5100.0 Total342,811100415,186100181,948100 Number and Percent of Trips per OD Pair Source: 1990 CTPP data for Commuters who live in the region, excludes workers at home. ACS Urban Test Counties in the NCHRP 8-48 Database Comparison of Key Data Issues in the Analysis Data Sets TableContent 1Total Workers (1) 2Means of Transportation (7) by Vehicles Available (3) 3Poverty Status (3) 4Minority Status (2) 5Household Income (8) 6Means of Transportation (17) 7Means of Transportation (4) by Household Income (4) 8Mean Travel Time by Means of Transportation to Work (7) and Time Leaving Home for Work (2) 9Median Travel Time by Means of Transportation to Work (7) and Time Leaving Home for Work (2) 10Aggregate Number of Vehicles by Time Leaving Home for Work (2) 11Number of Workers per Vehicle by Time Leaving Home for Work (2) 12Aggregate Number of Carpools by Time Leaving Home for Work (2) 13Number of Workers per Carpool by Time Leaving Home for Work (2) 14Aggregate Travel Time by Means of Transportation to Work (7) and Time Leaving Home for Work (2) Part 3 – Worker Flow Tables with Disclosure Rules No record threshold Must have 3 unweighted records What is Rounding? For confidentiality considerations the Census Bureau rounded all CTPP 2000 tables except for those containing means, medians, and standard deviation values. The rounding rules were simple. Values of zero remained zero. Values between 1 and 7 were rounded to 4. And values of 8 or more were rounded to the nearest multiple of 5. Rounding Analysis Plan Take 1990 un-rounded CTPP data, apply the 2000 rounding rules and examine the affects of rounding on the data. How does rounded and un- rounded data compare? Was rounding values between 1 and 7 to 4 a good decision? Did the rounding rules have a different affect on different geographical Summary Levels? What are Thresholds? The threshold rule stated that no data would be provided for any Origin- Destination pair that had 3 or less records (trips) before weighting. The Analysis Plan Take data CTPP2000 and ACS test data for three areas and compare the number of trips/workers and the number of OD pairs with Thresholds and without. Data Used for Analysis  CTPP2000 Part 3 Tables 3-01 and 3-06  ACS NCHRP 8-48 Test Data Part 3 Tables 3-01 and 3-03  Workers who lived in and worked in study area including those working at home  Three study areas or Counties Area and Summary Level Rounding Rule of Seven Total Commuters Lost Commuters Percent Lost Chicago, IL (TAZ) Without3,487,23200.00 With3,342,963144,2694.14 Los Angeles, CA (Tracts) Without6,657,85700.00 With6,505,471152,3862.29 Boston, MA (Block Groups) Without2,022,51900.00 With1,941,61280,9074.00 Rounding 1.Produces Inconsistencies Among CTPP Table Values 2.Caused a Systematic Undercount of Workers 3.Did not Show a Significant Noticeable Difference on Summary Levels 4.Rounding to 5 Would Have Been Better 5.Was Not Well Received by Users Thresholds 1.Eliminates Most of the OD Pairs and Commuters 2.Renders the Flow Data Useless 3.Undermines the Utility of Small Area Data 4.Was Not Well Received by Users Boston Block Group Summary 0 0 2 2 4 4 6 6 8 8 10 12 14 16 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Number of Trips per OD Pair Percent of All Trips Can You Find the Midpoint? — It is not 4! 40 30 20 10 1 1 2 2 3 3 4 4 5 5 6 6 7 7 Taz Tract BG Taz Tract BG Source: 1990 Part 3 CTPP. Data Used for Analysis  1990 Part 3  Resident Workers (commuters who lived in region), excluding those who worked at home  Three Regions (Chicago, Los Angeles, and Boston)  Three Summary Levels or small area geographies (TAZs, Tracts and Block Groups) Number of Trips per OD Pair Percent of All Trips Source: NCHRP 8-48 Test Dataset Tables Workers Lost Due To Rounding and Thresholds Source: CTPP2000 Part 3 and ACS NCHRP 8-48 Test Data Part 3. Part 1 – At Residence (121 Tables) All Tables Rounded Zero = 0 1 through 7 = 4 8 through ∞ = Nearest Multiple of 5 Part 2 – At Workplace (68 Tables) All Tables Rounded Part 3 – Worker Flows (14 Tables) All Tables Rounded Some Tables with Thresholds 1990 Commuters Lost Due to Rounding