这个问题我已答了几次了，呵呵，看来这个命令还很有用啊。
h xtbalance
打开的页面中点第一个网址链接，然后安装。
命令格式：
xtbalance, range(numlist) [ miss(varlist) ]
选项：
range(numlist) specifies sample range to be transfored. numlist must be two integers and specified in ascending order.
miss(varlist) forces to drop the observations if any one of the variable in varlist has missing value
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