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).
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