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st: reshaping data?


From   agostino@unical.it
To   statalist@hsphsun2.harvard.edu
Subject   st: reshaping data?
Date   Thu, 25 Oct 2012 15:51:11 +0200

Dear all,
I would be grateful if you could help me to address the following problem:
I have to estimate the equation

Q1=a+b1P1+b2P2+?bnPn+e

where Q1 is quantity of item 1 sold by a supermarket during a week , P1 is the price of item 1 in that week, the other prices are those of the n items belonging to the same family of items. My data set is organized as follows:

Code_item	week	Q	P	Code_family
789		1	8	2	1
790		1	25	1	1
791		1	9	1	1
792		1	12	2	1

800		1	7	4	2
801		1	20	1	2
802		1	11	3	2
803		1	12	2	2

And so forth for the other weeks and families?

For each item, how can I include in my regression the prices of the other (n-1) items of the same family, ignoring the prices of the items belonging to other families?
Thank you in advance
Maria


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