第一：你说的空缺值应该是missing value   ，在stata中它就等于.
第二：变量范围大于0，应该没有包含缺失值的。
第三：最后一个你自己加一下就可以看到结果了，应该是回到那个数值。
有不对的地方还请见谅

谢谢。

试了一下，好像是数字-缺失值=缺失值，崩溃了

看Stata的帮助missing values的解释啊。
这个不能猜啊。
Stata里面有27种缺失值表示，

Title

    [U] 12.2.1 Missing values
Description
    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).


Remarks

    More details concerning missing values and their treatment in Stata are provided under the following headings:

        Overview
        Expressions
        Operators
        Functions
        Matrices
        Useful commands
        Value labels
        Estimation commands
        Technical note:  Version 7 and earlier


    Overview

    1.  Stata supports different types of numeric missing values that can be used to specify different reasons that
        a value is unknown.  The most frequently used missing value ., referred to as sysmiss, is nearly always
        generated by Stata when it cannot assign a specific value.  The 26 extended missing values .a, .b, ..., .z
        are available to users requiring more elaborate tracking of missing values.

        Empty strings are treated as missing values of type string.

    2.  Numeric missing values are represented by large positive values.  This means that an expression such as
        income > 100 evaluates to true for missing values of the variable income, as well as to those that are
        greater than 100.  Also, the simple expression if varname evaluates to true for all nonzero values of
        varname, including missing values.

    3.  The ordering of missing values is

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

    4.  Most Stata statistical commands deal with missing values by disregarding observations with one or more
        missing values (called "listwise deletion" or "complete cases only").

stata的数据文件中，缺失值的表现形式并不只有“.”，一般的还有“.d”“.e”....“.r”，类似的都是空缺值，如果不了解数据中的数值形态，而是自己默认所有空缺值都是“.”，那么在处理空缺值（比如drop if VAR==.）的时候就会漏掉很多。