How to easily define a constant variable in a panel in Stata

Question

I have a panel dataset (country-year) in Stata. For instance I have GDP in 1990,1991,..2010 for many countries.

I want to define a variable "GDP in 2006" which exists for all years and contains the 2006 value of GDP.

The way I am doing it now works but is a but clumsy so I was hoping someone would have a better idea:

   qui gen gdp2006=.

   replace gdp2006=gdp if year==2006

   forval t=2007/2010 {
       sort country year
       qui replace gdp2006=gdp2006[_n-1] if year==`t'&country[_n-1]==country
   }
   forval t=2005(-1)1990 {
       sort country year
       qui replace gdp2006=gdp2006[_n+1] if year==`t'&country[_n+1]==country
   }

Thanks!

Answer 1

You can do this in one line

egen gdp2006 = mean(gdp / (year == 2006)), by(country) 

(year == 2006) evaluates as 1 or 0, so the expression

gdp / (year == 2006)

evaluates as gdp when year is 2006 and missing otherwise. Missings are ignored in calculating the mean for each country.

For a wider and more systematic discussion see http://www.stata-journal.com/article.html?article=dm0055

P.S. The techniques you know permit shortening of your code:

gen gdp2006 = gdp if year == 2006
bysort country (gdp2006): replace gdp2006 = gdp2006[_n-1] if _n > 1