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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: Merge chaniging census tracts |

Date |
Thu, 5 May 2005 21:22:22 +0100 |

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. * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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