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Fwd: st: Count no of unique occurrences in past 12 months


From   "Marietta Jones" <marjones75@googlemail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Fwd: st: Count no of unique occurrences in past 12 months
Date   Thu, 31 Jul 2008 18:43:39 +0100

Sorry, ignore the previous message; there was a problem with the formatting.
Please have a look at this one.


---------- Forwarded message ----------
From: Marietta Jones <marjones75@googlemail.com>
Date: Thu, Jul 31, 2008 at 6:36 PM
Subject: st: Count no of unique occurrences in past 12 months
To: statalist@hsphsun2.harvard.edu


Dear Statalist users,

I am a novice stata user and I have a problem calculating a variable.

I have the following dataset:

Company_no       Date                Investor_id
1                     03/01/2000                 1
1                     04/09/2000                 1
1                     12/12/2001                 2
2                     13/12/2000                 4
2                      07/08/2001                7
3                      09/08/2000                4
3                      19/03/2001                4
3                      02/05/2001                5
3                      03/12/2001                6


For each observation, I would like to calculate the number of distinct
investors owning shares in a given company in the previous 12 months.
The new variable (No_investors_past_12_months) will look as follows:

Company_no       Date                Investor_id     No_investors_past_12_months
1                     03/01/2000                 1
             1
1                     04/09/2000                 1
             1
1                     12/12/2001                 2
             1
2                     13/12/2000                 4
             1
2                      07/08/2001                7
             2
3                      09/08/2000                4
             1
3                      19/03/2001                4
             1
3                      02/05/2001                5
             2
3                      03/12/2001                6
             3

For example, for company 2 on 07/08/2001 there are two investors who
have owned shares of this company within the past 12 months.

The idea is that the same investor should be counted only once and we
should count all observations over the past 12 months, including the
current observation date. If there are no observations over the past
12 months, the value of the new variable should equal 1.

I would appreciate any help with this problem.
Thank you.


Marietta
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