If an observation has a missing value for a response (VAR) variable, PROC NPAR1WAY excludes that observation from the analysis. Any observation with a missing or nonpositive value for the FREQ variable is also excluded from the analysis.

By default, PROC NPAR1WAY also excludes observations with missing values of the CLASS variable. If you specify the MISSING option, PROC NPAR1WAY treats missing values of the CLASS variable as a valid class level and includes these observations in the analysis.

PROC NPAR1WAY treats missing BY variable values like any other BY variable value. The missing values form a separate, valid BY group.