| year | firm | attribute |
2010 | 001 | a |
2010 | 002 | b |
2010 | 002 | a |
2011 | 001 | b |
2011 | 001 | a |
2012 | 002 | a |
2012 | 003 | b |
| year | firm | attribute | number |
2010 | 001 | a | 1 |
2010 | 002 | b | 2 |
2010 | 002 | a | 2 |
2011 | 001 | b | 2 |
2011 | 001 | a | 2 |
2012 | 002 | a | 1 |
2012 | 003 | b | 1 |
by year firm: egen number=count(continent)
但是,不成功,也不知道为啥。
希望各位多多赐教!


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