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st: Creating a new variable with information from other observations


From   "Davide Cantoni" <davide.cantoni@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Creating a new variable with information from other observations
Date   Sun, 18 May 2008 14:22:10 +0200

Hello,

I am having a rather intricate problem in creating a new variable in a
panel dataset, and I appreciate any help you could offer. I hope the
problem can potentially be of general interest.

I have a panel dataset of cities and their characteristics in
different countries. I know the latitude of each one of these cities,
but now I want to create an additional variable reflecting the
latitude of the capital city of the country a given city lies in. So
for example: for the cities of New York, Chicago, etc., I want this
new variable to contain the latitude of Washington, DC.

Here is a description of the dataset's structure: it is a panel in
long form, with cities in different countries, observed over different
years. Each city has a unique numeric identifier, "cityid". Then there
is a country identifier, called "countryid". Finally, there is a
variable that repeats the capital city's cityid for each city in a
given country, "capitalid". For instance, if the cityid of London was
135, all cities in the dataset that are in the UK would get a value of
135 in the variable "capitalid". Finally, there is a variable called
"latitude" that refelcts the latitude of each city.

How would I now proceed to create this new variable, call it
"latitude_capital", by using the variables above?

Basically, the problem I'm having is
- tell stata to look up for each city its capitalid
- browse the dataset until you find a city that has the cityid equal
to this capitalid
- find out the latitude of this capital city
- go back to the original city and replace "latitude_capital" with the
latitude you've just retrieved

The additional problem I encounter while trying to construct something
with "foreach..." (that, at least, is what I was trying so far) is
that the values that the capitalid variable takes are of course not a
clean numlist (like "1(1)100"), but rather a sequence of numbers
without any regularity, such as 11 12 50 54 60 131... and so on.
Thanks for your suggestions!

Davide Cantoni
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