根据 Nick Cox 之建议：

仔细看看stata的帮助，缺失值是代表很大的正值。
所以你写的命令结果是没有问题的，问题是不了解缺失值到底是多大。


Stata has 27 numeric missing values:

        ., the default, which is called the "system missing value" or sysmiss

    and

        .a, .b, .c, ..., .z, which are called the "extended missing values".

    Numeric missing values are represented by large positive values.  The ordering is

                             all nonmissing numbers < . < .a < .b < ... < .z

谢谢您的解答，非常感谢。已解决~

我明白了！感谢您的回复！您说的缺失值代表很大的正值也正好印证了这两句话：
  Caution: If the first argument to cond() is a logical expression, that
                     is, cond(x>2,50,70,.), the fourth argument is never reached.