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st: Merge changing census tracts


From   Julia Gamas <jgamas@MIT.EDU>
To   statalist@hsphsun2.harvard.edu
Subject   st: Merge changing census tracts
Date   Fri, 6 May 2005 11:21:33 -0400

Dear all,
the equivalence table in my example is structured as follows:

year 1 year 2
Code   code  what happened to tract over time:

xx     lm    census tract from year 1 split into two 
xx     pq    "
pl     pl    census tract remained but got bigger because
da     pl    census tracts from year 1 merged into an existing
gr     pl    tract
fa     fa    census tract remained the same
00     no    census tract did not exist in year 1 but appeared in year
00     lp    "
ll     00    census tract existed in year 1 but disappeared in year 2
pp     00    "

Is this the information you need?
Thank you for your help.

Julia

Date: Thu, 5 May 2005 21:22:22 +0100
From: "Nick Cox" <n.j.cox@durham.ac.uk>
Subject: st: RE: Merge chaniging census tracts

Evidently there is an equivalence table that tells 
you that the mapping is 

   xx         -> lm, pq 
   pl, da, gr -> pl         
   fa         -> fa 
              -> no 

   etc. 

independently of the two files. You need to 
tell us how that is structured, I believe. 

Nick 
n.j.cox@durham.ac.uk 

Julia Gamas

> I have to merge two databases from two different years where 
> each observation is
> a census tract. The challenge is avoiding double counting 
> after the merge, as a
> result of census tracts changing through time. I would like 
> to create a code
> that identifies the groups of tracts that split up or merged 
> but am not sure
> how to do it.  Other than doing it by hand (I have 4000 
> tracts), I'm wondering
> if there is a somewhat automatic way to do this in Stata.
> Currently, if I were to merge the two datasets using the
> corresponding equivalence table I would get something like this.
> year 1     year 2
> Code pop  code  pop  what happened to tract over time:
> xx    22  lm     10  census tract from year 1 split into two 
> xx    22  pq     5   "
> pl    12  pl     5   census tract remained but got bigger because
> da     5  pl     5   census tracts from year 1 merged into an existing
> gr     1  pl     5   tract
> fa     8  fa     16  census tract remained the same
> 00     0  no     10  census tract did not exist in year 1 but 
> appeared in year
> 2
> 00     0  lp     8   "
> ll    10  00     0   census tract existed in year 1 but 
> disappeared in year 2
> pp    10  00     0   "
> 
> If at this point I were to collapse (sum) the population for 
> year 2, for
> example, then I would end up triple counting population in 
> census tract pl,
> similarly, if I were to collapse (sum) population for year 1. 
>  Population for
> pl in year 2 would be counted as 15 when it is only 5.  
> Population for tract xx
> would be counted as 44 when it is only 22.  As an 
> intermediate step I was
> thinking of doing the following:
>          year 1     year 2
> newcode  Code  pop code  pop  
> 1        xx    22  lm     10  
> 1        xx    22  pq     5   
> 2        pl    12  pl     5   
> 2        da     5  pl     5   
> 2        gr     1  pl     5   
> 3        fa     8  fa     16  
> 4        00     0  no     10  
> 5        00     0  lp     8   
> 6        ll    10  00     0    
> 7        pp    10  00     0   
> 
> That way, I could keep track of the population for each tract 
> or "group" of
> tracts and end up with
> something like this:
>          year 1   year 2
> newcode  pop      pop  
> 1        22        15 
> 2        18         5      
> 3        8         16   
> 4        0         10 
> 5        0          8
> 6        10         0  
> 7        10         0
> How do I tell stata how to generate newcode (i.e. can I get 
> stata to recognize
> groups of census tracts with the same code and assign them a 
> newcode, for both
> years)?
> Any help would be appreciated.

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