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:  checking if a value is missing


    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").