The following example illustrates how SAS propagates missing values. That is, for some calculations, SAS assigns a variable a missing value if any of the values contributing to the calculation are missing. In this example, SAS generates missing values when attempting to calculate the volume of the tree when either the height or the circumference is missing:

OPTIONS PS = 58 LS = 72 NODATE NONUMBER;

DATA trees;
    input type $ 1-16 circ_in hght_ft crown_ft;
	volume = (0.319*hght_ft)*(0.0000163*circ_in**2);
	DATALINES;
oak, black        222   . 112
hemlock, eastern  .   138  52
ash, white        258  80  70
cherry, black     187  91  75
maple, red        210  99  74
elm, american     229 127 104
;
RUN;

PROC PRINT data = trees;
    title 'Tree data';
RUN;

First, review the data and note that the first tree is missing a value for hght_ft and the second tree is missing a value for circ_in. Then, launch and run   the SAS program, and review the log window to see the "missing values generated" message SAS displays in this situation. If you review the output from the PRINT procedure:

you can see that SAS did indeed assign a missing value to the volume variable for the first two observations.

When you are working with a large data set, it can be difficult to locate all the places in which a missing value was generated based on a calculation. In that case, you'll probably want to use a selecting IF statement to find the missing values. The following example illustrates using an IF statement to find the observations that are assigned a missing value for the newly calculated variable volume:

OPTIONS PS = 58 LS = 72 NODATE NONUMBER;

DATA trees;
    input type $ 1-16 circ_in hght_ft crown_ft;
	volume = (0.319*hght_ft)*(0.0000163*circ_in**2);
	if volume = .;
	DATALINES;
oak, black        222   . 112
hemlock, eastern  .   138  52
ash, white        258  80  70
cherry, black     187  91  75
maple, red        210  99  74
elm, american     229 127 104
;
RUN;

PROC PRINT data = trees;
    title 'Trees with Missing Volumes';
RUN;

First, note that the only thing that differs between this program and the previous one is the presence of the IF statement. Then, launch and run   the SAS program, and review the output from the PRINT procedure to see the two observations for which the volume is deemed missing.