David and Kelly--
Just in case my last post wasn't crystal clear, here is an example
using faked data and a quick-n-dirty distance calculation (if I've
done this right, the -merge- approach does not depend on having more
patients than hospitals, or vice versa):
clear
range pid 1 10 10
set seed 123
g p_lat=32+uniform()*10
g p_lon=-120+uniform()*40
save /patients, replace
clear
input id cntrl VA h_lat h_lon
0785 45 1 42.103 -80.065
0150 45 1 39.739 -75.605
0960 45 1 37.499 -77.470
0510 45 1 39.464 -77.962
0030 45 1 27.844 -82.796
0625 45 1 39.138 -84.509
0570 45 1 38.271 -85.694
0020 45 1 36.143 -86.805
1260 33 0 35.832 -86.073
9061 33 0 36.313 -87.676
0305 45 1 33.505 -86.801
0365 33 0 32.288 -90.258
0470 45 1 41.628 -93.659
0006 45 1 44.412 -103.469
0004 45 1 40.807 -96.625
0493 33 0 35.608 -91.265
0070 45 1 31.292 -92.461
0126 45 1 31.080 -97.348
1310 33 0 31.783 -106.474
0180 45 1 46.612 -112.048
0060 45 1 43.618 -116.194
0085 33 0 41.242 -110.991
0540 45 1 34.568 -112.456
0075 33 0 40.698 -111.990
0190 33 0 40.044 -111.716
1180 45 1 46.053 -118.356
0009 33 0 33.860 -118.149
1538 33 0 34.093 -118.344
end
isid id
save /hospitals, replace
desc using /hospitals, sh
local nh=r(N)
use /patients
local np=_N
g d2va=.
g id4va=.
g d2nva=.
g id4nva=.
merge using /hospitals
forv i=1/`np' {
local x1=p_lat[`i']
local y1=p_lon[`i']
local x2 h_lat
local y2 h_lon
tempvar L
local R=6367.44
qui {
g double L=(`y2'-`y1')*_pi/180
replace L=(`y2'-`y1'-360)*_pi/180 if L<. & L>_pi
replace L=(`y2'-`y1'+360)*_pi/180 if L<-_pi
local t1 acos(sin(`x2'*_pi/180)*sin(`x1'*_pi/180)
g d=`t1'+cos(`x2'*_pi/180)*cos(`x1'*_pi/180)*cos(L))*`R'
g va_d=d if VA==1
g nva_d=d if VA==0
scalar dv=va_d[1]
scalar iv=.
scalar dnv=nva_d[1]
scalar inv=.
forv j=2/`nh' {
if va_d[`j']<dv {
scalar dv=va_d[`j']
scalar iv=id[`j']
}
if nva_d[`j']<dnv {
scalar dnv=nva_d[`j']
scalar inv=id[`j']
}
}
replace d2va=dv in `i'
if dv<. replace id4va=iv in `i'
replace d2nva=dnv in `i'
if dnv<. replace id4nva=inv in `i'
}
drop L d va_d nva_d
}
drop if _m==2
drop id cntrl VA h_lat h_lon _m
l
la var d2va "Distance to nearest VA hosp"
la var d2nva "Distance to nearest non-VA hosp"
la var id4va "Hosp id for nearest VA hosp"
la var id4nva "Hosp id for nearest non-VA hosp"
*
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* http://www.stata.com/support/faqs/res/findit.html
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* http://www.ats.ucla.edu/stat/stata/
