如果纪录第一个 事件 （STATA)

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有一些公司的合作事件及其发生时间。时间是在1990-2009。 有些公司可能有多起合作事件。
如何纪录 那些公司  在 3年内只有一次 合作事件。

For example,
Company A established a joint venture with Company B on Jan 5, 1991; (joint venture 1)
Company A estalbished another joint venture with Company C on Feb 9, 1993. (joint venture 2)
Company A established the third joint venture with COmpany D on Dec 5, 1994. (joint venture 3)
Company A establisehd the fourth joint venture with Company E on Dec 6, 2005 (joint venture 4)

Then I have to record the first and the fourth joint venture.
I should not record the second and the third one because the second one is within 3 years of the first one; and the third one is within 3 years of the second one.

How can I use the STATA command to record these two joint ventures?

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关键词：Stata tata establish ventures Venture 公司 合作 another example venture

假设 date为时间变量，

sort firm date

bys firm: gen num_year=(date[_n-1]-date[_n])/365.5

gen jointcase=(num_year>3)