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From: david_a._vandenbroucke@hud.gov on 9/30/2003 2:07:22 PM
Subject: AHS Metro Areas

I would appreciate any thoughts you might have about how to define metropolitan
areas in the American Housing Survey, particularly the metro surveys. Should
we maintain a fixed geography, change to match whatever OMB definition is
current, or try to find a compromise between these?

Since the AHS is a longitudinal survey, one point of view would be to freeze
the geography at the time we draw the sample. Subsequent surveys would then
apply only to that region. It would be easy to use a series of surveys to
examine how that area changed over time. If the original geography matched the
then-current OMB definition, it would include the core of the metropolitan area
in any case. We wouldn't have to worry about confidentiality problems caused
by changing the geographic definitions, and users would be less confused about
exactly what bits of ground were covered in each year. The disadvantage is
that the survey would not cover any fringe counties that were added over the
years. Tabulations from the AHS would reflect only the core of the metro
area. Thus, the survey would be good at answering questions about a specified
patch of ground but less useful to answer them about currently defined
metropolitan areas.

The alternative is to modify the AHS geography to track changes in the official
metro area definitions. This would involve drawing new samples in any areas
added to the definitions. Since new areas are typically sparsely populated at
the time they are added, we will not usually have enough new population
(100,000 or more persons) to define new zones. Thus, the new areas will have
to be added to existing zones. Because of longitudinality and confidentiality,
zones assignments cannot be changed later. In some situations (such as now,
when OMB is changing the way it defines metropolitan areas), parts of the old
metro area may have to be deleted. In order to do this, all cases in the
affected zones will have to be dropped, and a new sample drawn for the new,
modified zones. Thus, longitudinality will suffer, even in those parts of the
zones that remain in the metro area. Changes from one AHS to the next will
reflect changes in geography as well as changes within the previous geography,
and users will not be able to isolate the two kinds of changes from one
another. Users interested in geography will have to carefully trace the
modifications over the years. On the other hand, tabulations from each survey
will accurately reflect the metro area as currently defined.

A side issue that also affects longitudinality is that, for budgetary reasons,
we are more-or-less freezing the sample sizes in the metro surveys. The metro
surveys have always been under pressure to keep costs down, and we feel that
the total sample sizes are currently large enough for most purposes. Since we
do need to add cases to reflect new construction, we have to eliminate some
existing cases to balance the new ones. We can compensate for this by changes
in the weighting, and the full sample will reflect the proportional
representation of newer and older units. However, this cost-saving strategy
does gradually erode longitudinality. At some point we may have to draw
entirely new samples. This is an expensive proposition, and I don't expect
that we will do this before the 2010 census results are available.

A more radical strategy would be to dispense with longitudinality for the metro
surveys and simply draw a new sample, based on current OMB definitions, each
time. We are not entertaining this idea at present, but it does have a certain
methodological purity.

Our current strategy is based on the belief that users are more interested in
having accurate tabulations for OMB-defined metropolitan areas than in
following changes to units in specified areas over time. Thus, we are
following the second method described above and working to conform the AHS
survey areas to the new Core Base Statistical Areas (CBSA). We'll be giving
you the details as they become relevant for each metro survey, beginning with
the 2006 survey.

Dav Vandenbroucke
Economist
U.S. Dept. HUD
david_a._vandenbroucke@hud.gov
202-708-1060 ext. 5890


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