nicely said Patty; as always. Alan
----- Original Message -----
From: "Patty Becker" <pbecker(a)umich.edu>
To: <ctpp-news(a)chrispy.net>
Sent: Wednesday, January 23, 2002 1:31 PM
Subject: [CTPP] Data detail
Let me try to address some of the issues that Liz
Hartmann raises:
Over the past two or three years, the concerns about privacy and
confidentiality in data release have grown exponentially. At the census
bureau, this issue exploded two years ago with the question of what would
be on the Public Use Microdata Sample file (PUMS). The original plan
would
have, essentially, made the file useless. After a
user meeting convened
by
the Population Division at the bureau, and a LOT of
internal negotiating,
the final outline for the 2000 PUMS file is quite reasonable. I have no
problem with top coding and rounding, because, in fact, the data are not
reliable at the per minute level for journey to work, or at the details of
very high income levels. My only problem is with the 100,000 cutoff, but
it was clear that we could not fight that this time around.
The long-form data are weighted to represent the total population in
publications and summary files. You can see those weights on the PUMS
file. Theoretically, they would be 6 to 1, since the sample is 1 in 6,
but
a lot of things can make them different from that. I
have not heard what
the cutoff level is going to be on SF4 for the long form data; the
critical
point is that we won't have any cutoff level on
SF3. Remember, again,
that
SF2 and SF4 are a completely different concept than
they were in
1980/90. They have exactly the same tables as SF1/SF3, but are iterated
for the small race/ethnic groups such as Indian tribes when there is a big
enough N to do so. That makes perfect sense to me.
The important point that needs to be made for TAZ files (especially the
TAZ
to TAZ) is that these files are used only in modeling.
No one makes a
printed (or web-based) report out of them! We all know, or should, that
the data are very, very thin, and thus really are not very reliable. But
there are so many TAZ pairs that the model handles the problems--or at
least I hope it does.
I think there is a reasonable chance that we will be able to get the CTPP
tables needed for transportation planning when the ACS reaches its full
five-year cycle, planned for 2008. The data will still be thinner than
long-form data; we will have a chance to measure what all that means for
geographic areas larger than tracts/TAZs as the one year, two year, and
three year accumulations become available. We are supposed to get
one-year
tables for areas of 65,000 or more, and two-year data
for areas of 30,000
or more. That's probably enough for the districts used in the models.
"Is there room for collapsing across some of the "people categories" in
order to have data for the smaller geographic levels? How much would that
cost in dollars and timeliness?" That's already handled in the design of
the tables for any given product, including the CTPP. The TAZ/TAZ file
has
nothing but the count of workers, right? We get, or
impute,
characteristics from the wider data set that's on the TAZ of residence and
TAZ of work files.
Yes, I completely agree with "But I'd hate to lose the baby with the
bathwater. There's so much to be gained from this incredible data source.
It's one of the best things going (in my opinion), and crucial to good
Planning." Personally, I plan to remain in the fight as long as
possible. We will have to deal with it product by product. Two weeks
ago,
I attended a DC conference on privacy and
confidentiality. Of perhaps 275
people there, at least 240 were from federal agencies. There were very
few
users. We have to make sure the user community is
heard from.
I will be happy to work with the CTPP community on this. I am a
demographer rather than a transportation planner, but have more experience
than most demographers with the CTPP (and UTPP) and the use of these data.
Patty Becker
--------------------------------------------------------------------------
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Patricia C. (Patty) Becker 248/354-6520
APB Associates/SEMCC FAX 248/354-6645
28300 Franklin Road Home 248/355-2428
Southfield, MI 48034 pbecker(a)umich.edu