Missing values for variables in programming statements are treated in the same way that missing values are treated in the DATA step; that is, missing values used in expressions propagate missing values to the result. See SAS Language: Reference for more information about missing values.
Missing values in the input data are treated as follows in the COMPUTAB report table. At the end of the input block, either one or more rows or one or more columns can have been selected to receive values from the program data vector (PDV). Numeric data values from variables in the PDV are added into selected report table rows or columns. If a PDV value is missing, the values already in the selected rows or columns for that variable are unchanged by the current observation. Other values from the current observation are added to table values as usual.