缺失值

和 . 有什么区别吗，为什么数据中.和.d都有

    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

    Thus, the expression age > 60 is true if variable age is greater than 60 or missing.

    To exclude missing values, ask whether the value is less than ".".  For instance,

        . list if age > 60 & age < .

    To specify missing values, ask whether the value is greater than or equal to ".".  For instance,

        . list if age >=.

    Stata has one string missing value, which is denoted by "" (blank).

也就是说这两者都是表示缺失值，但.<.a<.b<.c等等，缺失值也有大小之分，一般.默认为正无穷，对不？

不同缺失值可能表示不同含义。得看你用的数据库是怎样来定义的。