I was reporting on the results from the NCHRP 08-79
report of ONE MPO in

the validation phase of the project.  What Ed has provided is nationwide

results from 2000. 

Elaine

-----Original Message-----

From: ctpp-news-bounces@chrispy.net

[mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Ed Christopher

Sent: Friday, March 11, 2011 7:06 PM

To: ctpp-news@chrispy.net

Subject: Re: [CTPP] How small is small ? Thoughts on TAZs

It might be more accurate to note that the median size of TAZs in 2000

was 619 and the largest one was 80,113.  Attached is a summary of
the

TAZ sizes for 2000.  Its somewhat interesting to look at the

distribution which is bi-modal.

Elaine's point is still valid but to suggest that the average TAZ was

over 1000 when the median was 600 tells me that the average might not be

the best metric given some extreme values.

Elaine.Murakami@dot.gov wrote:

> Many of you will be working on TAZ and TAD delineations, starting
on 

> March 16.  March 16 is the day the Census Bureau Geography office

> hopes to have the software and User Guide available, and for many

states to

> have the geographic files for downloading.   If you are part
of the

TAZ

> delineation program, you will get a email directly from the Census

> Bureau with the password information.

> 

>  

> 

> I have been reading the NCHRP 08-79 Task 6 "validation phase"
report.

> (The August 2010 CTPP Status Report includes an article about this

> project http://www.fhwa.dot.gov/ctpp/sr1008.htm ) You will recall
that

> this project is developing a method to create partially synthetic

> microdata ACS records to use in the CTPP 5-year (2006-2010)

tabulation.

> This is necessary otherwise, most of the tables with the variable

> "means of transportation to work" would be suppressed.  That
is, for 

> this round of the CTPP,  "partially synthetic data"
was determined to 

> be preferred to data suppression for small geography.

> 

>  

> 

> Only SOME of the CTPP tables are deemed to be risky for individual

> disclosure, so only SOME of the CTPP tables will have to use the 

> synthetic microdata records.  The other CTPP tables will use
the 

> actual ACS microdata records.

> 

>  

> 

> If you attended the TAZ training webinars (2/25 and 2/28), and if
you 

> reviewed the TAZ software specifications, you know that the software

> WILL allow for very small TAZs, but the software will warn you if
the 

> resident or estimated worker counts are below 600.  In the NCHRP
08-79

> validation phase, the actual ACS data was compared to the synthetic

> ACS in the protected environment at the Census Bureau.  For a
medium 

> sized MPO in the test, the MPO's new TAZ system for their regional

> model has very small TAZs (on average only 300 residents per TAZ),
and

in

> CTPP2000, the average was 1000 per TAZ.   Although the synthetic
data

> performed as well as the actual ACS in model output runs, Westat found

> that the "ACS cell sample size will create data usability issues
for 

> transportation planners at fine levels of geography, e.g. TAZs, for

> cross-tabulations of key variables with means of transportation."

> 

>  

> 

> So, you have a choice.  For defining TAZs for CTPP, you can make

> larger TAZs than for your model and have less data perturbation, or

> you can make very small TAZs and have a lot of data perturbation.

> 

>  

> 

> Elaine Murakami

> 

> FHWA Office of Planning

> 

> 206-220-4460

> 

>  

> 

>  

> 

> 

> 

> 

> ----------------------------------------------------------------------

> --

> 

> _______________________________________________

> ctpp-news mailing list

> ctpp-news@chrispy.net

> http://www.chrispy.net/mailman/listinfo/ctpp-news

--

Ed Christopher

708-283-3534 (V)

708-574-8131 (cell)

FHWA RC-TST-PLN

4749 Lincoln Mall Drive, Suite 600

Matteson, IL  60443

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