st: what command to keep firms-years with multiple occurrence of only one type of events

[Nahla Betelmal <nahlaib@gmail.com>]

Dear Statalist members,

It would be great if you can let me know what command I can use to
keep firms-years with multiple occurrence of only one type of events
in my data.

I have firms-years-events data set. A firm can have multiple entries
for the same year if it undertakes certain corporate events . I coded
the events as 1,2,3, 4. For example the data looks like this


Firm   Year    Event
A        1994       2
A        1994       3
A        1994       3
A        1995       1
A         1995      3
A         1996       2
A         1996       4
A         1996       3
A          1997       3
A           1997      3
A           1998       1
A            1998       1
B
..


I am only interested in studying firm-years where  only event 3 was
undertaken (i.e. keep both entries for year 1997 only). In other
words, I want to drop firm years where there are a mix of events shown
as multiple entries/observations ( years 1994, 1995, 1996) and
multiple unique occurrence of event other than 3 (year 1998).

I understand that I can do it on two stages, first drop  firm-years
with mix events, then second stage drop years with unique events other
than 3.

the problem that I  don't know what command should I use to do the
first stage ( drop firms-years with multiple entries of multiple
events).

Thank you in advance, I highly appreciate your kind help

Nahla Betelmal
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