st: atribute values between lines of a variable/standardize data

[Lucas Ferreira Mation <lucasmation@gmail.com>]

Hello StataListers,

I´m stuck with this:
My data is divided into several groups, with many observations in
each. For each group, I want to "standardize" my data based on a
specific subset of observations (in this case, divide the actual
values of Y by the means of a specific subgroup of Y). How can I do
that?
In the example bellow, for each group, I need to "standardize" the
values of Y based on the average of Y of the years 2003 and 2004. I
managed to create such means for those observations, but I don´t know
how to extend that value to the rest of the observations of that
subgroup.

"
input year str20 group Y
2001 G1  57
2002 G1  61
2003 G1  54
2004 G1  60
2005 G1  64
2001 G2  1543
2002 G2  1700
2003 G2  1532
2004 G2  1659
2005 G2  1800
end
egen denominator=mean(Y) if(year==2003 | year==2004), by(group)
*this creates the desired mean(denominator for the "standardization")
*but only for the observations in years 2003 and 2004.
*how do I attribute that to the rest of the observations in that
group? Having this, I would run:
gen Y_standardized=Y/denominator

"
In my actual data is quite long with many groups and many observations
(months) per group.
Any sugestions would be greatly appreciated
thanks in advance
Lucas
IPEA-Brasil

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/