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https://stats.idre.ucla.edu/stata/faq/how-can-i-fill-downexpand-observations-with-respect-to-a-time-variable/
您即将离开知乎,请注意您的帐号和财产安全。
https://stats.idre.ucla.edu/stata/faq/how-can-i-fill-downexpand-observations-with-respect-to-a-time-variable/