Is there a way to combine these variables in a way that makes sense? - gis

Question

Hello stack overflow community!
I am a sociology student working on a thesis project comparing home value appreciation and neighborhood racial composition over time.
I'm currently using two separate data sources and trying to combine them in a way that makes sense without aggregating anything.
The first data source is GIS data which has information on home sales in each year by home. The second is census data which has yearly estimates of racial composition by census tract. Both are in .csv formats.
My goal is to create a set of variables for each home row in the GIS data which represents the racial composition for the tract the home is in at the year it was sold (e.g. home 1 | 2010| $500,000 | Census tract 10 | 10% white).
I began doing this by going into Stata and using the following strategy:
For example, if I'm looking at a home sold in 2010 in Census tract 10 and I find that this tract was 10% white in 2010, using something like
If censustract=10 and year=2010, replace percentwhite = 10
However, this seemed incredibly time consuming, as I'm using data that go back decades and a couple dozen Census tracts.
Does anyone have any suggestions on how I might do this smarter, not harder? The first thought I had was to aggregate the data by census tract and year, but was hoping to avoid that if possible. Thank you so much in advance for your help and have a terrific day and start to the new year!

Answer 1

It sounds like you can simply merge census data onto your GIS data. That will be much less painful than using -replace-. Here's an example:
*GIS data: information on home sales in each year by home
clear
input censustract house_id year house_value_k
10 100 2010 200
11 101 2020 500
11 102 1980 100
end
tempfile GIS_data
sa `GIS_data'
*census data: yearly estimates of racial composition by census tract
clear
input censustract year percentwhite
10 2010 20
10 2000 10
11 2010 25
11 2000 5
end
tempfile census_data
sa `census_data'
*easy method: merge the census data onto your GIS data
use `GIS_data', clear
mer m:1 censustract year using `census_data'
drop if _merge==2
list
*hard method: use -replace-
use `GIS_data', clear
gen percentwhite=.
replace percentwhite=20 if censustract==10 & year==2010
replace percentwhite=10 if censustract==10 & year==2000
replace percentwhite=25 if censustract==11 & year==2010
replace percentwhite=5 if censustract==11 & year==2000
list
Both methods "work", but using -merge- is much easier and less prone to errors.
Note: I intentionally created the data sets so that the merge wouldn't be perfect. You will likely want to drop some of the observations in that case. In the code above I dropped when _merge==2