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BASICS REGRESSION T-TEST ANOVA CORRELATION

SPSS System Variables

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 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

*1. Create test data.

data list free/age.
begin data
14 19 29 30 31
end data.

*2. Use $casenum in regular command.

compute first_case = $casenum eq 1.
exe.

*3. Convert $casenum to normal variable.

compute id = $casenum.
exe.

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 Current Date and Time SPSS $time

SPSS Syntax Example 2

*1. Create single case test data.

data list free/now.
begin data
0
end data.

*2. Compute now = current date and time.

compute now = $TIME.
exe.

*3. Display now as datetime variable.

formats now(datetime22).

Final Note

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

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