Missing data
If you are not using column input or column pointers, you may need to specify that a particular value
in a dataset is missing. SAS regards a single period (.) as a missing numeric value and a blank space
as a missing character value (only in column input). SAS also uses these conventions when printing
data or results of calculations. When SAS analyzes data with missing values, any observations with
missing values of variables required in the analyses are automatically excluded from consideration.