st: filling missing rows

[demariaf@unical.it]

Dear All,
My database includes the following variables:
-	Country-pair
-	Product lines
-	Year
-	Bilateral Trade flows;
-	GDPs for exporter  and importer countries;
-	Preference tariffs;


The database treats as missing the observations where trade flows are  
zero.  I need to recover such zero values, and at the same time I have  
to preserve the value of the other variables. Trying to explain with  
an example, suppose having  the following country pair : EU-ALBANIA  
and EU-BARBADOS and  suppose  the total number of  agricultural  
product lines is 6 (actually they are hundreds, I�m  just simplifying  
to be  clear). Now,  in 2000 no lines is missing,  by contrast in 2001  
some product lines are missing as follows:

Country pair    Lines       Year Trade flows GDP_IMP GDP_EXP  
Preference tariffs
EU-ALBANIA	1	2000	50	100	85	2
EU-ALBANIA	2	2000	60	100	85	3
EU-ALBANIA	3	2000	100	100	85	2
EU-ALBANIA	4	2000	90	100	85	1
EU-ALBANIA	5	2000	60	100	85	1
EU-ALBANIA	6	2000	40	100	85	7
EU-BARBADOS	1	2000	60	100	90	3
EU-BARBADOS	2	2000	100	100	90	2
EU-BARBADOS	3	2000	90	100	90	1
EU-BARBADOS	4	2000	60	100	90	1
EU-BARBADOS	5	2000	40	100	90	7
EU-BARBADOS	6	2000	40	100	90	6
EU-ALBANIA	1	2001	50	110	87	2
EU-ALBANIA	2	2001	90	110	87	1
EU-ALBANIA	4	2001	20	110	87	5
EU-BARBADOS	1	2001	50	110	92	2
EU-BARBADOS	6	2001	60	110	92	3

Instead I should have the following complete data for 2001

Country pair    Lines       Year Trade flows GDP_IMP GDP_EXP Preferencetariffs
EU-ALBANIA	1	2001	50	110	87	2
EU-ALBANIA	2	2001	0	110	87	3
EU-ALBANIA	3	2001	0	110	87	2
EU-ALBANIA	4	2001	90	110	87	1
EU-ALBANIA	5	2001	0	110	87	1
EU-ALBANIA	6	2001	40	110	87	7
EU-BARBADOS	1	2001	60	110	92	3
EU-BARBADOS	2	2001	0	110	92	2
EU-BARBADOS	3	2001	90	110	92	1
EU-BARBADOS	4	2001	0	110	92	1
EU-BARBADOS	5	2001	0	110	92	7
EU-BARBADOS	6	2001	0	110	92	5.5

I have added  extra  rows so as to obtain  6 lines each year for each  
pair of countries.  Please notice that I need only the trade flows to  
be zero, whilst the other variables have to maintain the same value  
they have in that particular year. Moreover, I need to impute  the  
right line codes to the new created rows. This is a difficult task, as  
there are so many different combinations of missing lines in different  
years (the total number of lines is almost 500...)
I gave a look to the command FILL , but it does not seem to solve my  
case. Do you know another  command or way, which enables to get what I  
need?
Thank you very much for any light!
Federica


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