From rhoskins@home.com Mon Apr 17 07:06:50 2000 Received: from mxu4.u.washington.edu (mxu4.u.washington.edu [140.142.33.8]) by lists.u.washington.edu (8.9.3+UW99.09/8.9.3+UW99.09) with ESMTP id HAA46072 for ; Mon, 17 Apr 2000 07:06:48 -0700 Received: from mail.rdc1.wa.home.com (imail@ha1.rdc1.wa.home.com [24.0.2.66]) by mxu4.u.washington.edu (8.9.3+UW00.02/8.9.3+UW99.09) with ESMTP id HAA04243 for ; Mon, 17 Apr 2000 07:06:48 -0700 Received: from c501552a ([24.5.121.123]) by mail.rdc1.wa.home.com (InterMail vM.4.01.02.00 201-229-116) with SMTP id <20000417140647.MQMZ7733.mail.rdc1.wa.home.com@c501552a> for ; Mon, 17 Apr 2000 07:06:47 -0700 Message-ID: <013401bfa876$2b456a40$7b790518@olmpi1.wa.home.com> From: "Dick Hoskins" To: References: <200004151521.LAA19848@smtpsrv1.isis.unc.edu> <38FB14B4.BDE489AB@sph.emory.edu> Subject: Re: WAPHGIS: rural versus urban Date: Mon, 17 Apr 2000 07:06:47 -0700 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2919.6700 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2919.6700 Yes I have found the same variability here in WA, i.e. urban > rural. In fact there are empty census tracts in urban areas but not in rural ones in WA. For Bayesian smoothing purposes, etc I have to check that adjacent census tracts are not empty - an empty census tract may not be relevant to a disease rate if it is always empty or very low pop because its an industrial zone, has an airport in it. One of the things I am trying to get a handle on is when does a tract become urban that was once clearly rural? Census tracts in the Snoqualmie Valley which was just cows 5 years ago are now full of Microsoft millionaires or wannabes who think they are living in a rural region. Zips are relative stable because the Post Offices figures out in a hurry if no one lives there are re-draws Zip Code "boundaries" (not a real entity I understand) to meet mail delivery needs. Census block groups are even worse with respect to variability. Some are very low pop or empty now that were full in 1990. This is not the case in rural areas. As I understood it, census tract boundaries were set up to have a somewhat equal population but to also reflect some socioeconomic homogeneity as well. Dick Hoskins rhoskins@home.com GIS uses in public health summer course: http://healthlinks.washington.edu/inpho/gis/course.html ----- Original Message ----- From: "Lance Waller" To: Sent: Monday, April 17, 2000 6:42 AM Subject: Re: WAPHGIS: rural versus urban > Thanks for the information on the Beale codes, very > interesting. > > I have a brief comment and question for the list regarding > possibly extending to (US) census tracts. > > I've analyzed some census tract level data from central > NY and have some impressions about tract data that I would > like the list to comment on as to their generality. > > Some background (as I understand it, corrections encouraged): > > - Tracts were originally designed to have roughly the same > population size in each (about 3000 people or so). Naturally, > this results in geographically small tracts in areas with > higher population density, and geographically large tracts > in areas with lower pop'n density. > > Now, (my data is 1980 census data) I naively expected to > find higher pop'n sizes in urban areas over time, but what > I find is much higher *variability* (big AND small numbers) > in pop'n size in "urban" > tracts than in "rural" tracts (I suppose due to rezoning, more > mobile population etc). The result is that the lowest pop'n > sizes often occur in "urban" tracts (in my case, a pop'n size > of 3 people in one tract in Syracuse), while the "rural" > tracts stay about the same size (less mobile and larger > geographically, so people have to move farther to get out > of the tract). Some rural tracts have small pop'n sizes, > but most of the small ones seem to be in the cities in my > data set. > > This is of interest to me since some spatial smoothing procedures > for rates argue about small numerators (pop'n sizes) in rural > tracts when the problem is actually often worse in urban tracts. > > Anyone else have this experience? > > (I expect ZIPs are a bit more "stable" (ironic use of the word as > ZIPs can change at any time) in pop'n size since they > are reassigned if they get too small or large (pop'n wise)). > > Lance Waller > Dept of Biostatistics > Rollins School of Public Health > Emory University > Atlanta, GA .