[CTPP] ACS 5-year Federal Register Notice

[Ed Christopher]

I am not sure if others are pondering what the recent Federal Register 
notice on ACS 5-year data products really means but I believe that there 
are some “key” facts missing.  Facts that would allow users’ the ability 
to asses the true impact of the rules.  For example:

Rule 7 of the Disclosure Avoidance rules states that “For the residence 
and workplace tables where means of transportation (mode) is crossed 
with one or more other variables, there must be at least three 
unweighted workers in sample for each transportation mode in a given 
place for the table to be released.  Otherwise the data must be 
collapsed or suppressed and complementary suppression must be applied. 
There is no threshold on univariate tables.”
http://www.census.gov/acs/www/Downloads/proposal_acs5yearproducts.pdf

While this might sound very straight forward we know from the 
“statistical collapsing” rules that were applied to the 3-year data that 
only one collapsing attempt is made of the data and if the table does 
not pass it is tossed out (suppressed).  What we do not know about the 
current rule is what will the collapsing hierarchy be, and will the 
Census continue collapsing modes into each other until the threshold is 
met?  Both the Federal Register notice and the rules are silent on this 
issue.

In some recent work done for FHWA by the Census Bureau we do know that 
at a Tract level a great deal of data will be suppressed under Rule 7. 
Using 5-year data from five of the ACS test counties, Multnomah, Lake, 
Broward, San Francisco and Bronx we do know that if all 17 modes (means 
of transportation) are used only 3 to 8 percent of the Tracts would have 
enough data to be released.  However, if we cut the number of modes down 
to only 4 including Drove Alone, Carpooled, Transit, and other including 
work at home somewhere between 5 and 37 percent of the tables will be 
suppressed.  And that is at the TRACT level!  If per chance someone 
wants to go one step up the modal ladder and split out “bike and walk” 
and make work at home a separate category yielding 6 modal categories, 
upwards of 80 percent of the TRACT level tables can expected to be 
suppressed.  At the Block Group the suppression will be only worse.

-- 
Ed Christopher
Resource Center Planning Team
Federal Highway Administration
19900 Governors Drive
Olympia Fields, Illinois  60461
708-283-3534 (V)  708-574-8131 (cell)
708-283-3501 (F)