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CTPP 2006-2010 Frequently Asked Questions

November 19, 2013

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Q1. How do I access the CTPP 2006-2010 data?

A. The CTPP 2006-2010 data are available through the on-line access software and the access link is http://ctpp.transportation.org/Pages/5-Year-Data.aspx

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Q2. Is registraion required to access the CTPP 2006-2010 data?

A. No. You can access the CTPP 2006-2010 data without registration, but to save your own geography selections and customized reports you MUST register. The registration is free to all users.

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Q3. How does the CTPP 2006-2010 data product differ from the ACS Standard data product?

A. Key differences are:

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Q4. How does the CTPP 2006-2010 data product differ from the CTPP 2006-2008 data product?

A. The CTPP 2006-2010 data product is based on 5 years of ACS data collected between 2006 and 2010; the CTPP 2006-2008 data product is based on 3 years of ACS data collected between 2006 and 2008. Other key differences are:

CTPP 2006-2010 data CTPP 2006-2008 data
It is available for small geographies including Census Tracts and TAZs. There is no population threshold. It is only available for geographies more than 20,000 population. There are a number of counties and cities for which data are not available.
It is a mix of modeled and actual data. It is all actual surveyed data.
There are three new tables crossed by Means of Transportation (Minority Status, number of workers in households, and Presence of Children). Means of Transportation crossed with Age of Worker, Time leaving/arriving, travel time, income, and vehicles available
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Q5. How does the CTPP 2006-2010 data product differ from the CTPP 2000 data product?

A. Key differences include:

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Q6. What is a Transportation Analysis District (TAD)?

A. TAD is a newly introduced concept for the CTPP 2006-2010. TAD has a population threshold of 20,000. The concept for adding TADs is that the ACS 3-year tabulation is limited to geographic areas with 20,000 population or over.

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Q7. Why are the TAZs in CTPP different from TAZ stuctures in the regional travel demand model?

A. TAZs in CTPP are called Census TAZs and they were delineated by MPOs and State DOTs for the CTPP tabulation purpose. Some regional models now use very small TAZ which would have no ACS samples for tabulation, therefore larger census TAZ were delineated.

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Q8. Why are some TAZs missing when I know it is a major employment center?

A. As of October 28, 2013, CTPP staff is working on a resolution to this problem, but it may not be resolved until December 2013. You may be missing some workplace TAZs that have zero residence population. For example, Pentagon TAZ in Washington, DC is currently missing in the Part 2 workplace data and Part 3 flows.

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Q9. What are the differences between Set A(from microdata) and Set B (from disclosure proofed microdata) tables for CTPP 2006-2010?

A. The CTPP 2006-2010 is a mix of modeled and actual data. Set A tables are based on non-disclosure proofed microdata (actual data) and Set B tables are based on disclosure proofed microdata (modeled data). Row and column totals do not match for the same population and geographies between Set A and Set B, but differences should be within 10%. Please refer to NCHRP 08-79: Producing Transportation Data Products from the ACS that Comply With Disclosure Rules report for details on disclosure proofing process utilized in the CTPP 2006-2010.

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Q10. How do the Census Bureau Disclosure Rules impact the CTPP 2006-2010 data?

A. The rounding rules apply to both Set A and Set B tables. For example, values are rounded to the nearest 5 (e.g. 263 is rounded to 265). The cell suppression rule (there must be 0 or at least 3 or more sample to show the table) and the table suppression rule (if any one cell in a table is suppressed, the whole table is suppressed) only apply to Set A non-disclosure proofed data. Please refer to the CTPP E-learning module - Disclosure Proofing ACS Data in CTPP for details on the Census Bureau disclosure rules. http://ctpp.transportation.org/Pages/elearningmodules.aspx

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Q11. Why don't the employment numbers from the CTPP match LODES?

A. The simple answer is that the data are different. CTPP uses ACS data, while Longitudinal Employment Household Dynamics (LEHD) Origin-Destination Employment Statistics (LODES) uses administrative records. The ACS is a collection of responses from a sample of residential addresses. CTPP is a custom tabulation of the ACS, which presents additional confidentiality issues as the number of cross tabulations increases. The LODES uses the Quarterly Census of Employment and Wages (QCEW), which is a list of employers and their employees for whom unemployment insurance coverage is provided. The QCEW may have a business address that is not the same as a workplace location. An estimated 10 percent of workers are self-employed and are not included in the QCEW. FHWA has compared the employment data among CTPP, LODES and the Current Population Survey (CPS), and the article can be accessed from http://www.fhwa.dot.gov/planning/census_issues/ctpp/status_report/sr100113.cfm.

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Q12. If the Margin of Error (MOE) is so high, why should one bother using the results?

A. If there is another data source that can provide reliable data for the geographic area, you may want to consider using that other source. However, if it is important to compare the results of different geographic areas to each other, one might still be better off using the ACS derived dataset since the survey methods have been applied consistently across all areas of the United States. Be sure to test the significance of any comparisons to ensure that they are statistically acceptable for your particular needs. It may also be important to make sure that the margin of error (MOE) is not larger than the midpoint or value of the original data. For example, if the number of people who walk to work is 326 and the MOE is 350 you should not use the data. For additional help, please visit the U.S. Census Bureau's ACS webpage: http://www.census.gov/acs/www/guidance_for_data_users/guidance_main/

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Q13. Why are there no tables on disability in CTPP 2006-2010?

A. The wording of the disability question had substantial changes between 2007 and 2008. Therefore, disability tables are limited to the ACS data for 2008 and onward.

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Q14. How do I calculate the MOE when I combine multiple areas?

A. There are several calculators and guides.

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Q15. Is there a resource on significance testing when comparing the CTPP 2006-2010 to 2000, 1990 or older Census results? Is it necessary to do any testing in these circumstances?

A. The sample sizes for the 2000 decennial Census Long Forms were much larger than the ACS, even after 5 years of accumulating ACS responses. The associated MOEs for CTPP 2006-2010 estimates are therefore larger in geographical areas containing relatively small sample sizes. To calculate the MOE for the Census 2000 long form results, please go to the ACS Statistical Analyzer project, created by The National Center for Transit Research (South Florida) http://www.nctr.usf.edu/abstracts/abs77802.htm

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Q16. How do I get the full data set for CTPP 2006-2010?

A. It is currently possible to get the full data set, which is 350 gigs uncompressed, by starting a request process by email to pweinberger@aashto.org. The full data set is unsupported and for power users, comfortable manipulating large data sets in statistical software packages that they are already licensed to use.

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Q17. Worker Counts in Small Area Workplace Geographies related questions

a: Why does the sum of workers within an area differ from the total worker count for that same area in Part 2 and Part 3?

A: For large geography (County and Place), workplace coding is completed for 100 percent of workers. For smaller geographies such as TAZs and Tracts, some extended workplace coding (using imputation) occurs, resulting in about 91 percent of national workplace coding for small geographies such as TAZs and Tracts. Some counties have no "extended" workplace coding. Workers with missing workplace location information are not counted as part of the total number of workers for small geographies. This results in differences between the sum of the total number of workers across Tracts and TAZs for some workplace geographies and the corresponding single estimate for a larger area that contains those geographies (e.g. counties).

b. How much difference can there be?

A: Nationwide, the standard geocoding process coded about 75% of workplace addresses to small geography, and the "extended" workplace allocation coded another 15 percent. Because extended workplace coding does not occur in all counties (see the list), the percent of workplace coding differs by geography. The difference between the sums at the county level and at smaller geographies within that county may vary considerably with the largest variation generally found among more rural counties.

c: What should I do to resolve the differences?

A: If there are differences in total workers for some geographies and the sum of workers in smaller geographies within the larger geography, then alternative local or national datasets may be used to supplement the worker information provided in the CTPP. Strategies to account for missing workers will vary according to data users’ needs, resources, and supporting information. Data users should use their understanding of the local area to guide decisions about if and how to allocate workers that are unaccounted for at small geographies.

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