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st: Panel Data Manipulation


From   Jason Hwang <jjhwang@fas.harvard.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: Panel Data Manipulation
Date   Fri, 4 Nov 2005 12:36:40 -0500 (EST)

Dear Statalisters,

I would appreciate help with the following panel data manipulation
problem. I have data of the form:

seller	year	product	buyer	volume
...
100	1990	1111	200	100
100	1991	1111	200	200
100	1992	1111	200	250
100	1990	2222	200	.
100	1991	2222	200	80
100	1992	2222	200	83
100	1990	2222	500	100
100	1991	2222	500	125
100	1992	2222	500	200
...

The dot in volume means sales volume in that year was zero. I would like
summarize the data by answering two questions. First, each seller sells
many products to many buyers. Let "firstyear" be the first year a given
seller sells anything to a given buyer. In the example above, for the
seller-buyer pair 100-200, "firstyear" would be 1990. Then for every other
product 100 sells to 200 in subsequent year, I want to calculate the gap
between year of first sale and "firstyear". I would like to generate a
table of the form:

seller	product buyer timegap	firstyear
...
100	1111	200	0	1990
100	2222	200	1	1991
100	2222	300	0	1990
...

Second, I would like to ask: for each seller, how much of the increase in
total sales of each product is driven by changes in sales to existing
buyers or sales to new buyers. I would like to generate a table of the
form:

seller	product year 	change	same 	new
...
100	1111	1991	100	100	0
100	2222	1991	105	25	80
...

Note that from 1990 to 1991, the total sales of 2222 increased by 105, of
which 25 came from increasing sales to an existing customer 500, and the
rest 80 came from selling to a new customer 200.

One complication is that sometimes a seller will make sales with a given
buyer in one year, make zero sales the next year, and generate positive
sales another year. In the case of this last year, I would like to count
the sales as coming from "same" customer.

I would very much appreciate your help. Thanks very much.

Jason
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