PROC FREQ has two options for specifiying how to handle missing values:
MISSPRINT: displays missing value frequencies in frequency or crosstabulation tables but does not include them in computations of percentages or statistics.
MISSING: treats missing values as a valid nonmissing level for all TABLES variables. Displays missing levels in frequency and crosstabulation tables and includes them in computations of percentages and statistics.
For more information on these options (and the SPARSE option with the LIST option), see
Base SAS(R) 9.4 Procedures Guide: Statistical Procedures, Third Edition
