Summary

SPSS uses a handful of hidden variables for keeping track of things. These so called system variables have names starting with "$" and can be useful in some cases. The most important one is $casenum, the number shown next to each case in the data editor window.

SPSS $casenum

SPSS $casenum

In the left outline of the data editor window (under Data View) we see a number next to each case. This number is internally known as $casenum. To some extent we can use $casenum in normal SPSS commands. We can also convert it into a regular variable. This can be used to create a unique row id if this is absent. The syntax below demonstrates this.

SPSS Syntax Example 1

SPSS Select If and $casenum

Using SELECT IF with $casenum can be tricky. This is because SPSS uses $casenum to keep track of cases during procedures. If cases are deleted during a procedure, their $casenum may shift to a different case. Like so, SELECT IF $casenum GT 1. will delete all cases in the data. What happens, is this

1. SPSS starts at the first case. Since its $casenum is not greater than one, it's deleted.
2. The second case now shifts up one row and is thus assigned $casenum 1.
3. Since the second case's $casenum is not greater than one (anymore), it's deleted as well.
4. Now the third case shifts up and gets $casenum 1. And so on.

SPSS $time

A second system variable that may sometimes be useful is $time. This is hold the current date and time. As with $casenum, it can be used straight away, for example in DATEDIFF. Alternatively, its value can be passed into a regular variable as demonstrated by the final syntax example.

SPSS $time

SPSS Syntax Example 2

Final Note

SPSS uses more system variables than $casenum and $time. Consult the command syntax reference for a complete overview.