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st: Variable Manipulation


From   "Katarina Sikavica" <k-sikavica@kellogg.northwestern.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Variable Manipulation
Date   Sat, 1 Mar 2008 15:49:16 +0100

Dear Statalisters:

I am working with a panel dataset containing company performance data
and would like to calculate 
a) the annual change in return on assets (varname: ROA) which is
calculated as e.g. ROA in the year 2000 - ROA in 2001
b) the ratio of return index (varname: RETIND), calculated as e.g.
RETIND in 2000 / RETIND in 1999

The dataset is a unbalanced panel of company performance data 1998-2005
and contains, unfortunately, quite some missings in the performance
variables (!). The missings are coded using "." The data can be grouped
according to company name, year (varname: YEAR) and a unique numeric
identfier(varname: DSCODE) retrieved frot Thomson Anaytics.

I have spent some time trying to generate the above mentioned variables
(I have tried playing around with _n and _N) but was, unfortunately
unable to obtain the correct variable. I have searched STATA FAQ and
have used the findit command but given that I don't even know what the
right keyword is I did not find any useful information. 
I appologize if this procedure has allready been explained elsewhere...

Many thanks for your help,
Best,
Kate 


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