Definitions for "Missing value"
Keywords:  omit, infer, null, sas, ignore
In a computer file for a survey, unanswered questions and those with answers like "Don't know" are declared to be missing values, and are usually excluded when calculating percentages etc. Sometimes, missing values can be guesstimated using imputation.
The value for an attribute is not known or does not exist. There are several possible reasons for a value to be missing, such as: it was not measured; there was an instrument malfunction; the attribute does not apply, or the attribute's value cannot be known. Some algorithms have problems dealing with missing values.
a value in the SAS System indicating that no data is stored for the column in the current row. By default, the SAS System prints a missing numeric value as a single period (.) and a missing character value as a blank space.