Subcommittee on Census Data For Transportation Planning

Issues to Consider in Determining Content of 2000 CTPP
Prepared by Phil Salopek, Elaine Murakami and Tom Mank
July 10, 1998 (revised 7/27/98)

Disclosure Avoidance and Average Cell Size

The Census Bureau Disclosure Review Board (DRB) will have a role in evaluating the tabulation requests for the CTPP. This was not the case in 1990. The most important issues for us are: For example, if a TAZ has 400 residents and 200 workers, a table with 40 cells, which counted workers, would have an average cell size of 5. The DRB has NOT determined a minimum average cell size for table definition. If you review the tables from 1990, you will find that some CTPP tables had over 200 cells. Unless the counts of residents and workers typically exceed something like 800 or 1000, we will not be able to request a table with 200 cells. We believe that the tables can be defined with fewer cells, without losing too much ability to analyze the data for specific geographic areas. One variable which we know is likely to cause problems is "means of transportation" because in most areas, modes such as subway, bicycle, taxi, ferry, are likely to be "0" (see item 12 below).

Phil Salopek and his staff will be making a presentation to the DRB on the CTPP in the near future. They are currently tabulating TAZ resident and worker populations from the 1990 CTPP for about 25 metropolitan areas.

Specific Topics

1).  Delete Parts D, E, and F from the Statewide Element. These parts contained more detailed tabulations than in Parts A, B, and C. The size cutoff for counties and places was 75,000+ persons in D-F, not 2,500 as in A-C. The D-F data were tabulated by place of residence, place of work, and residence by work, respectively.

2).  Delete Part 4 from the Urban Element. This was a single table consisting of a cross-tab of Household Size by Units in Structure by Vehicles Available by Household Income. It was shown for the CTPP region as a whole, the Study Area(s), Urbanized Area(s), MSA(s)/CMSA(s), and PMSA(s).

3).  Delete Part 7 from the Urban Element. This was a set of 31 tables containing data on the characteristics of people working in census tracts in selected places (those with sufficient tract-coding rates). Part 7 was always tract-based, even in areas that used TAZs in the rest of the CTPP. Other geographic levels shown were US, State, county, place, and metropolitan areas.

After 1990 CTPP, Census Bureau did develop special tract-to-tract tabulations which were very popular. We may want to consider this type of tabulation as part of the CTPP.

4).  Delete Part 6 from the Urban Element. This set of eight tables was produced only for MPOs whose regions contained a million persons or more. Part 6 contained origin by destination data for super-districts, which were aggregates of 100,000 or so persons built up from the smallest units (TAZ, tract, or BG). Other geographic units included the CTPP region, study area, MSA/CMSA, and PMSAs.

5).  Delete Part 8 from the Urban Element. These tables were produced only for MPOs whose regions contained a million persons or more. Part 8 contained origin by destination data for TAZs, tracts, or BGs and was requested by the "modelers." There was basically a table of means of transportation by household income and one crossing mode by vehicles available. Other summary levels included CTPP region, study area, and central business district.

Regarding Items 1 -5. Since we are strongly considering the deletions of these several parts, we believe that a custom tabulation package that will allow 3- and 4-way tabulations will better serve our needs. The geographic unit would be larger than a TAZ, and perhaps even larger than a tract. Historically PUMS has permitted these types of tabulations using a sample of the long form responses (1% or 5% samples), it may be possible to develop a custom tabulation package for the complete sample of long form responses.

6).  In general, decrease the number of cells in the largest tables.  Disclosure avoidance procedures may dictate an average cell size.

7).  Increase, in general, the number of tables with data for Race/Hispanic Origin groups. Also, reach an agreement on the Race/Hispanic categories to be used, and decide if any multiple race breakdowns are wanted.

8).  Decrease, in general, the number of tables with data by sex.

9).  Decrease the number of income and earnings categories used.  Especially when crossed by other variables with large numbers of categories.

10).  Decrease the number of departure time categories used.

11).  Alter the disability tables, based on the changes in these questions on the 2000 form. In addition, look at decreasing the number of age categories used in the cross of disability.

12).  It may be worthwhile to divide the means of transportation into two mode choice groups:
        a) summarized list for selected areas having little public transportation (drive alone/car pool/public transporatation) and
        b) a detailed list for those with multi-faceted public transportation choices (alone/carpool /bus/trolley/taxi/railroad, etc).

Summarizing small response categories may aid in avoiding situations where data would be suppressed. Consolidating means of transportation into fewer categories would diminish "0" counts, thus increasing the average cell size (which may turn out to be a suppression criterion). FTA could provide a list of areas where multiple modes are possible. MPOs would have to ask to be put on the list if not already there.

Summarized List (12 Detailed List (18)

Total Total
Drove alone Drove alone
Carpool ­ 2 Carpool ­ 2
Carpool ­ 3 Carpool ­ 3
Carpool ­ 4+ Carpool ­ 4
Transit (all) Carpool ­ 5 or 6
Taxi Carpool ­ 7+
Motorcycle Bus/trolley bus
Bicycle Streetcar/trolley car
Walked Subway/elevated
Other means Railroad
Worked at home Ferryboat
Other means
Worked at home

13).  Are the data needed for standard transportation models used by all the states included? We know some states (e.g., Florida) have been looking at making some changes. Here are the tables from 1990.

1-13 Household size (5) by Number of Workers in Household (6)
1-14 Number of Workers in Household (6) by Household Income (26)
1-17 Household Size (5) by Vehicles Available (9)
1-18 Number of Workers in Household (6) by Vehicles Available (9)
1-20 Vehicles Available (9) by Household Income (26)

14).  The "Hours worked last week" item is no longer on the census questionnaire for 2000.  Should the tables which utilized this data (Part 2/B table 9, Part 1/A tables 28 and 32) be deleted, or do we want similar tables using the "usual hours worked per week, last year" item?

15).  Continue to include the unweighted sample counts of persons and housing units? These items are useful in evaluating data quality (e.g., calculating standard errors) and it is felt we should continue to provide them even if they are not widely used.

16).  Potential new tables:

Part 2/B
household earnings by means of transportation
time of arrival by occupation
time of arrival by industry
means of transportation by occupation
means of transportation by industry
vehicles available per worker
household income per worker by means of transportation

Part 3/C
household income per worker by means of transportation

17).  Ordering or sequencing of the tables within each part. The logic used in 1990 is not apparent and should be changed and/or explained and documented so things are easier to find. For example, put all the control total tables up front, both pop and housing, instead of putting all the housing tables, including the controls, together in the back of Part A/1.

Send you comments to either Ed Christopher or Tom Mank

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