SPSS Programming and Data Management, 4th Edition: A Guide for ...

15Best Practices and Efficiency TipsIn a similar fashion, if the set of transformations includes a command that selects asubset of cases and deletes unselected cases (for example, SELECT IF), lags will becomputed after the case selection. You will probably want to avoid case selectioncriteria based on lag values—unless you EXECUTE the lags first.Using $CASENUM to Select CasesThe value of the system variable $CASENUM is dynamic. If you change the sort orderof cases, the value of $CASENUM for each case changes. If you delete the first case,the case that formerly had a value of 2 for this system variable now has the value 1.Using the value of $CASENUM with the SELECT IF command can be a little trickybecause SELECT IF deletes each unselected case, changing the value of $CASENUMfor all remaining cases.For example, a SELECT IF command of the general form:SELECT IF ($CASENUM > [positive value]).will delete all cases because regardless of the value specified, the value of $CASENUMfor the current case will never be greater than 1. When the first case is evaluated, it hasa value of 1 for $CASENUM and is therefore deleted because it doesn’t have a valuegreater than the specified positive value. The erstwhile second case then becomes thefirst case, with a value of 1, and is consequently also deleted, and so on.The simple solution to this problem is to create a new variable equal to the originalvalue of $CASENUM. However, command syntax of the form:COMPUTE CaseNumber=$CASENUM.SELECT IF (CaseNumber > [positive value]).will still delete all cases because each case is deleted before the value of the newvariable is computed. The correct solution is to insert an EXECUTE command betweenCOMPUTE and SELECT IF, asin:COMPUTE CaseNumber=$CASENUM.EXECUTE.SELECT IF (CaseNumber > [positive value]).