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Create a sample dataset

Setting the height value to null to demonstrate _freq_ variable

 
data class02;
	set sashelp.class;
	if age=16 then do;
  		height=.;
	end;
run;

Code to demonstrate the difference between _freq_ and 'n' statistic

_FREQ_ variable in the output dataset presents the number of records contributed
'n' statistic represents the number of observations with non-missing value in a particular analysis variable
'nmiss' statistic represents the number of observations with missing value in a particular analys variable
In the output dataset, _freq_ indicates the total number of observations in class02 dataset
'n' statistic for height variable indicates a value of 18 as there are only 18(out of total 19) records with non-missing height value
'n' statistic for weight variable indicates a value of 19 as all 19 observations have a non-missing value

proc means data=class02;
	var height weight;
	output out=statsfreq2
	n=nheight nweight
   nmiss=nmissheight nmissweight
	mean=meanheight meanweight
	median=medianheight medianweight;
run;