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CTPP 2006-2008 Frequently Asked Questions (2011)

May 2011


Q: When will the 5-year CTPP with small area geography be available?

A. The 5-year CTPP (2006-2010 ACS records) including small area data will be delivered to AASHTO from the Census Bureau by mid-2013.

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Q. How does the CTPP 2006-2008 data product differ from CTPP 2000 data product?

A. The CTPP 2000 data product utilized data from the 2000 decennial Census "long form". The CTPP 2006-2008 data product uses responses to the ACS, a continuous monthly survey sample that provides annual estimates. This survey sample is much smaller than the Census 2000 and has restrictions on the geographic scale of tabulations (population threshold of 20,000 must be met) due to confidentiality issues and appropriate regulations from the Census Bureau's Disclosure Review Board (DRB). There are a number of counties and cities for which data are not available in the 2006-2008 3-year CTPP data product.

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Q. How do I access the CTPP 2006-2008 data ?

A. The CTPP 2006-2008 data are available through the AASHTO CTPP webpage http://ctpp.transportation.org/

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Q. Is there a fee for CTPP data?

A. There is no fee at the present time. The data are paid for under the sponsorship of the CTPP program. The CTPP is a custom tabulation of ACS data, under the sponsorship of many State Departments of Transportation, Metropolitan Planning Organizations, and the U.S. Department of Transportation. The Transportation Research Board (TRB), which is part of the National Academy of Sciences, is also a contributor.

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Q. Why don't the employment numbers from the CTPP, the numbers from the standard Census data products or those from the LEHD OnTheMap agree?

A. The simple answer is that the data are different. CTPP uses ACS data, while LEHD OnTheMap uses other data sources. The ACS is a collection of responses from a sample of residential addresses. The ACS is a small sample of the U.S. and estimates are available in 1-year, 3-year , and 5-year tabulations. The intention of the ACS is not to provide a yearly census but rather to collect a sample that is representative of the U.S. as a whole. Because the sample sizes are small in some geographies, the sampling error may be higher than Census 2000. CTPP is a custom tabulation of the ACS, which presents additional confidentiality issues as the number of cross tabulations increases. To comply with DRB rules, values are rounded to the nearest 5 (e.g. 263 is rounded to 265). The LEHD OnTheMap 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. Other workers, including the federal workforce, are also missing from the LEHD OnTheMap.

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Q. If my area has a large population and there is 1-year ACS, 3-year ACS, 3-year CTPP, and 5-year ACS results, which data table should I use?

A. It depends on which table you want, because some tables are ONLY available in the CTPP, and others are ONLY available in ACS. If your area exceeds the minimum population threshold (65,000) to have 1-year ACS estimates, you might want to try that, since it will use the most recent population estimates for sample weighting. But if your area is close to the 65,000 population threshold, it might be better to use a 3-year table from ACS or CTPP to reduce the Margin of Error (MOE) by having a larger sample.

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Q. 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 has 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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Q. Why are there no tables on disability in CTPP 2006-2008?

A. The wording of the disability question had substantial changes between 2007 and 2008. Therefore, until 2008-2010 can be combined into a 3-year tabulation, disability tables are limited to the 1-year ACS for 2008 and onward.

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

A. There have been several calculators and guides .

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Q. Is there a resource on significance testing when comparing the ACS 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 ACS 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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Q. What are some of the key features that users should know when using the ACS or the CTPP from ACS?

A. Three of the differences are:

  1. The ACS survey is conducted in all twelve months of the year, so the differences between the ACS and Census 2000 may be greater in areas with "snow bird" winter migrations, large college and university populations, for example.
  2. The ACS sample after 5 years of accumulation is about one-half the sampling rate of the Census 2000 "long form", which may result in larger MOEs.
  3. The ACS questionnaire only allows for one response for a workplace and one response for the commute mode. Therefore, people who may work at more than one location during the week have to select one place; and people who use 2 transportation modes to commute (e.g. drive to park & ride and then take a bus) are instructed to select the mode with the longest distance.
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Q. Should there be any special considerations when comparing ACS 2009 and ACS 2008 single-year data?

A. Certain year-to-year comparisons are of more concern to data users than others due to differences in sample weighting techniques . The 2009 (1-Year) ACS was the first data product released by the U.S. Census Bureau that implemented sub-county population estimates in the sample weighting methodology. Previously, the U.S. Census Bureau used TOTAL county estimates in the sample weighting - resulting in some estimates that seemed unreasonable. This was found in some large urban centers, such as Oakland, CA or Detroit, MI. By incorporating city estimates into the weighting process for the 2009 (1-Year) ACS, the results are anticipated to have fewer outliers. For example, 2008 ACS total population for Detroit was estimated as 777,500, and in 2009 ACS, was estimated as 911,000, for a difference of more than 130,000. For more detail about which geographies are impacted the most please read this Census document: http://www.census.gov/acs/www/Downloads/data_documentation/2009_release/ResearchNoteSubcountyControls.pdf

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Q. What other travel behavior data resources are available?

A. The 2008-2009 National Household Travel Survey (NHTS) can provide trips by purpose and mode, without geographic detail. The NHTS on-line table generator is an easy way to prepare custom tables from this survey of over 150,000 households. http://nhts.ornl.gov/index.shtml Using the 2001 NHTS, tract-level estimates were created in the "Transferability Project".

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