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Re: st: Constructing assignment var for Spatial RDD: For each id in region 1 need shortest distance to an id in region 0


From   Austin Nichols <austinnichols@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Constructing assignment var for Spatial RDD: For each id in region 1 need shortest distance to an id in region 0
Date   Sat, 28 Nov 2009 22:22:31 -0500

Chris <statachris@gmail.com> :
The distance to the nearest village in another region is not the
distance to the border; see the conversation with Laura Platchkov
(e.g. http://www.stata.com/statalist/archive/2009-09/msg00493.html
etc.) on how to compute distance to the nearest vertex on a polygon
from a shapefile that gives the boundary between regions.  This is
still not exactly the distance to the border, but it is closer.
Computing actual distance to the border would be a lot more work, but
is an interesting programming challenge.  And you really need that
distance measured accurately in your proposed regression discontinuity
design.

On Fri, Nov 27, 2009 at 5:49 PM, Stata Chris <statachris@gmail.com> wrote:
> Dear Statalisters,
>
> I would like to construct the assignment variable for a Spatial RDD,
> i.e. for each village in either of two regions I would like to know
> the distance to the border.
>
> Now, while I know the x- and y-coordinate of each village in the
> sample, I do not know the exact position of each point on the border;
> So my idea was to compute for each village in region 1 (0) the
> distance to each village in region 0 (1), as
> d=sqrt[ (y_1-y_0)^2  +  (x_1-x_0)^2  ],
> and then take for each the minimum of these as the value for the
> assignment variable. Because both regions have quite a few villages, I
> will need to compute many such distances, so I think it will probably
> be best to do this with a loop. I have a rough idea of what the loop
> should do, but I'm not entirely sure how exactly to communicate this
> to Stata.
>
> I am attaching the dataset, and below I provide a rough indication of
> what I think are the necessary steps. If one of you was able to help
> me put this into a syntax Stata does actually understand, that would
> be awesome:
>
> gen id1 = id if treatment==1
> gen id0 = id if treatment==0
> for each id1 {
>    for each id0 {
>       gen dist_id1_id0 = sqrt(  (zkx{id1}-zkx{id0})^2   +
> (zky{id1}-zky{id0})^2 )
>       gen assignmentvar_id1 = min (of all dist_id1_*)
>       }
>    }
>
> And then do the same also the other way round.
>
> Thank you so much,
> Chris
>

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