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Re: st: reshaping data?


From   agostino@unical.it
To   statalist@hsphsun2.harvard.edu, Nick Cox <njcoxstata@gmail.com>
Subject   Re: st: reshaping data?
Date   Thu, 25 Oct 2012 16:39:48 +0200

dear Nick,
thanks for your prompt reply. Unfortunately, the number of items is not fixed
best regards
Maria


Quoting Nick Cox <njcoxstata@gmail.com>:

This model formulation makes me feel a bit queasy, but I think what
you want is something like this.

Suppose for concreteness the number of items is fixed at 8. (I don't
see how this will work if the number is not fixed.) So, "for any value
of 8"

sort code_family code_item
forval i = 1/8 {
       by code_family : gen P`i' = P[`i']
}

Note that, contrary to your title, there is no -reshape- here as you
want your observations to remain observations; at least that's my
understanding.

Nick

On Thu, Oct 25, 2012 at 3:08 PM,  <agostino@unical.it> wrote:

I have to estimate the equation

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

Where Q1 is the 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      4       1
791             1       9       1.3     1

792             1       12      2       1

800             1       7       2       2
801             1       20      1.2     2
802             1       11      1.6     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?

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