27 Jan
2002
27 Jan
'02
4:03 p.m.
Block group data is and always has been important to the public sector. It
has been essential to my activities as a transportation planner over the
years. I cannot speak for other areas, but there is nothing "accidental"
about the geography of block groups in the areas in which I work. Perhaps
this is due to my active interest and involvement in the delineation of all
census statistical areas affecting my work. The necessity of meeting
minimum goals of persons and households for the sake of statistical analysis
does occasionally lead to somewhat arbitrary aggregations of small areas.
The necessity of delineating the block group boundaries prior to the actual
census does occasionally lead to odd-looking results, especially in rapidly
developing or redeveloping areas.
Errors made by the Bureau of the Census in field review, in digitizing, or
in geocoding are the only cause of "accidental" geography in my experience.
For the 1990 census, the Bureau would not allow local entities to demand
corrections to Bureau errors below the Census Tract level. In 2000, they
are allowing us to demand corrections that affect block groups, but not
individual blocks. (My metropolitan area had about half a mile of a major
drainage feature eliminated during field review. The Bureau admitted it was
their mistake but would not restore the block boundary.) In short, I'll
fight over any decisions that diminish my flexibility in performing
demographic analyses and would encourage other to do the same.
Robert R. Allen, AICP
Transportation Planning Director
Abilene Metropolitan Planning Organization
ph.: 915-676-6243
fax: 915-676-6242
Excerpt from Original Message-----
From: owner-ctpp-news(a)chrispy.net [mailto:owner-ctpp-news(a)chrispy.net]On
Behalf Of Patty Becker
Sent: Sunday, January 27, 2002 12:04 PM
To: ctpp-news(a)chrispy.net
Subject: [CTPP] Some more points
I am skeptical about block group data. They have never been very reliable
anyhow, and are mostly important to the private sector because they permit
more precise aggregation in what I call radius analyses (1 mile circle,
three mile circle, etc.) The long form data at the individual block group
level have very high sampling and non-sampling error rates. In addition,
block groups are largely "accidental" geography, because the lines between
them are most often arbitrary and not based on any real-life
criteria. Tracts (and TAZs), on the other hand, are deliberately drawn to
meet local criteria, or they should be.
I think it would serve us all better to refer to TAZs as tract-equivalents
rather than block-group equivalents. In areas without high employment
density, that's what they are. I don't think a fight over block group data
will serve anyone well as this point.
27 Jan
27 Jan
5 p.m.
New subject: [CTPP] Some more points
Robert,
If you actually delineate block group boundaries, you are a rare
exception. Yes, I do delineate some of them locally, primarily when tracts
have been combined in the City of Detroit (to maintain data for the old
tracts), or when there is a unique residential land use (such as a public
housing project) which is not large enough to be a tract. I understand
that in a smaller metro area, such as Abilene, you probably have more areas
which need such special treatment. However, I would venture to say that at
least 95% of the BGs in the nation are "accidental," where the BG lines
have been drawn by the Census Bureau's computer with no human intervention.
However much needed, block group data are basically very, very shaky. At
500 housing units per block group (a rough maximum; most are smaller), you
have only 50 cases in a 1 in 6 sample, to say nothing of having fewer
because of vacancies, non-response to the long form, and item
nonresponse. The sampling error on a distribution of 50 cases is roughly
+/- 14 percentage points, if memory serves me right. (I think it's 14 on
an N of 50 and 7 on an N of 100.) The non-sampling errors make the data
worse. That's why I recommend not using individual block group data.
Of course, I come from a large metro area with reasonably high density. I
can draw tracts which work so that we don't need block group data. That's
also why the TAZs are more like tracts than blocks. I know that it's much
harder when you have a smaller population base and still want to work at
the small area level. Just keep those error levels in mind, though.
Patty Becker
At 03:03 PM 01/27/2002 -0600, you wrote:
Block group data is and always has been important to
the public sector. It
has been essential to my activities as a transportation planner over the
years. I cannot speak for other areas, but there is nothing "accidental"
about the geography of block groups in the areas in which I work. Perhaps
this is due to my active interest and involvement in the delineation of all
census statistical areas affecting my work. The necessity of meeting
minimum goals of persons and households for the sake of statistical analysis
does occasionally lead to somewhat arbitrary aggregations of small areas.
The necessity of delineating the block group boundaries prior to the actual
census does occasionally lead to odd-looking results, especially in rapidly
developing or redeveloping areas.
Errors made by the Bureau of the Census in field review, in digitizing, or
in geocoding are the only cause of "accidental" geography in my experience.
For the 1990 census, the Bureau would not allow local entities to demand
corrections to Bureau errors below the Census Tract level. In 2000, they
are allowing us to demand corrections that affect block groups, but not
individual blocks. (My metropolitan area had about half a mile of a major
drainage feature eliminated during field review. The Bureau admitted it was
their mistake but would not restore the block boundary.) In short, I'll
fight over any decisions that diminish my flexibility in performing
demographic analyses and would encourage other to do the same.
Robert R. Allen, AICP
Transportation Planning Director
Abilene Metropolitan Planning Organization
ph.: 915-676-6243
fax: 915-676-6242
Excerpt from Original Message-----
From: owner-ctpp-news(a)chrispy.net [mailto:owner-ctpp-news(a)chrispy.net]On
Behalf Of Patty Becker
Sent: Sunday, January 27, 2002 12:04 PM
To: ctpp-news(a)chrispy.net
Subject: [CTPP] Some more points
I am skeptical about block group data. They have never been very reliable
anyhow, and are mostly important to the private sector because they permit
more precise aggregation in what I call radius analyses (1 mile circle,
three mile circle, etc.) The long form data at the individual block group
level have very high sampling and non-sampling error rates. In addition,
block groups are largely "accidental" geography, because the lines between
them are most often arbitrary and not based on any real-life
criteria. Tracts (and TAZs), on the other hand, are deliberately drawn to
meet local criteria, or they should be.
I think it would serve us all better to refer to TAZs as tract-equivalents
rather than block-group equivalents. In areas without high employment
density, that's what they are. I don't think a fight over block group data
will serve anyone well as this point.
-------------------------------------------------------------------------------------------------
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
28 Jan
28 Jan
10:25 a.m.
New subject: [CTPP] Some more points
Patty..
I must correct your misunderstanding in the statement below.
The Census Bureau has NEVER used computer algorithms to delineate block
groups (BGs). Computer algoritms have only been used to determine the
blocks and to number the blocks. Even for the 1990 census where there were
BNAs, it was the geographic staff in the Regional Offices who delineated the
BNAs and BGs, albeit with an eye towards field operations in the counties
with BNAs, not data tabulations. In 1990, the participants who worked on
census tracts also were allowed to suggest changes to the BGs for 1990.
For Census 2000, the participant statistical areas program
asked/allowed/encouraged all participants, nationwide, to review and suggest
changes to the the census tracts (including the former BNAs) and BGs. Some
participants did review and suggest changes to the BGs, many did not. Where
participants did not change the BGs, the geographers in the Regional Offices
made any necessary changes in accordance with the guidelines. The
guidelines used by participants are on the web at:
http://www.census.gov/geo/www/statarea.pdf
I would not equate, in any fashion, TAZs to census tracts. The purposes are
much different -- for TAZs, the goal is to get zones of approximately equal
trip ends so that the trip distribution models work better. For census
tracts, the goal is to maintain comparability over time, with a
minimum/ideal number of people with generally the same demographics. Thus
for TAZs, ideally there would be many, small TAZs in the business district
(for the work trip ends) where the CTs would be large because of minimal
residential population, while the comparable number of trip ends in a
residential area might cover several CTs.
Bob LaMacchia
Geography Division
U. S. Census Bureau
-----Original Message-----
From: owner-ctpp-news(a)chrispy.net [mailto:owner-ctpp-news(a)chrispy.net]On
Behalf Of Patty Becker
Sent: Sunday, January 27, 2002 5:00 PM
To: ctpp-news(a)chrispy.net
Subject: RE: [CTPP] Some more points
Robert,
If you actually delineate block group boundaries, you are a rare
exception. Yes, I do delineate some of them locally, primarily when tracts
have been combined in the City of Detroit (to maintain data for the old
tracts), or when there is a unique residential land use (such as a public
housing project) which is not large enough to be a tract. I understand
that in a smaller metro area, such as Abilene, you probably have more areas
which need such special treatment. However, I would venture to say that at
least 95% of the BGs in the nation are "accidental," where the BG lines
have been drawn by the Census Bureau's computer with no human intervention.
However much needed, block group data are basically very, very shaky. At
500 housing units per block group (a rough maximum; most are smaller), you
have only 50 cases in a 1 in 6 sample, to say nothing of having fewer
because of vacancies, non-response to the long form, and item
nonresponse. The sampling error on a distribution of 50 cases is roughly
+/- 14 percentage points, if memory serves me right. (I think it's 14 on
an N of 50 and 7 on an N of 100.) The non-sampling errors make the data
worse. That's why I recommend not using individual block group data.
Of course, I come from a large metro area with reasonably high density. I
can draw tracts which work so that we don't need block group data. That's
also why the TAZs are more like tracts than blocks. I know that it's much
harder when you have a smaller population base and still want to work at
the small area level. Just keep those error levels in mind, though.
Patty Becker
At 03:03 PM 01/27/2002 -0600, you wrote:
Block group data is and always has been important to
the public sector. It
has been essential to my activities as a transportation planner over the
years. I cannot speak for other areas, but there is nothing "accidental"
about the geography of block groups in the areas in which I work. Perhaps
this is due to my active interest and involvement in the delineation of all
census statistical areas affecting my work. The necessity of meeting
minimum goals of persons and households for the sake of statistical
analysis
does occasionally lead to somewhat arbitrary
aggregations of small areas.
The necessity of delineating the block group boundaries prior to the actual
census does occasionally lead to odd-looking results, especially in rapidly
developing or redeveloping areas.
Errors made by the Bureau of the Census in field review, in digitizing, or
in geocoding are the only cause of "accidental" geography in my experience.
For the 1990 census, the Bureau would not allow local entities to demand
corrections to Bureau errors below the Census Tract level. In 2000, they
are allowing us to demand corrections that affect block groups, but not
individual blocks. (My metropolitan area had about half a mile of a major
drainage feature eliminated during field review. The Bureau admitted it
was
their mistake but would not restore the block
boundary.) In short, I'll
fight over any decisions that diminish my flexibility in performing
demographic analyses and would encourage other to do the same.
Robert R. Allen, AICP
Transportation Planning Director
Abilene Metropolitan Planning Organization
ph.: 915-676-6243
fax: 915-676-6242
Excerpt from Original Message-----
From: owner-ctpp-news(a)chrispy.net [mailto:owner-ctpp-news(a)chrispy.net]On
Behalf Of Patty Becker
Sent: Sunday, January 27, 2002 12:04 PM
To: ctpp-news(a)chrispy.net
Subject: [CTPP] Some more points
I am skeptical about block group data. They have never been very reliable
anyhow, and are mostly important to the private sector because they permit
more precise aggregation in what I call radius analyses (1 mile circle,
three mile circle, etc.) The long form data at the individual block group
level have very high sampling and non-sampling error rates. In addition,
block groups are largely "accidental" geography, because the lines between
them are most often arbitrary and not based on any real-life
criteria. Tracts (and TAZs), on the other hand, are deliberately drawn to
meet local criteria, or they should be.
I think it would serve us all better to refer to TAZs as tract-equivalents
rather than block-group equivalents. In areas without high employment
density, that's what they are. I don't think a fight over block group data
will serve anyone well as this point.
----------------------------------------------------------------------------
---------------------
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
8588
days inactive
8589
days old
ctpp@listserv.transportation.org
2 comments
3 participants
participants (3)
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Allen -
Patty Becker -
Robert LaMacchia