This is not always true. Sometimes sas interpretes missing values as true. Example

data A; input x @@; if x then Expr="True "; else Expr="False"; datalines; 1 0 . proc print noobs; run;

data b;set a;y=x;if y<=0 then y=10;run;proc print data=b;run;

Obs x Expr y

1 1 True 1
2 0 False 10
3 . False 10
In this case, the second observation (x=0) is true (as it is <=0) and is given a value of y=10, but the third observation is also considered as true (because it is given a value of y=10) albeit the variable is missing.