SPSS-Computing New Variables

1) Using a wide variety of mathematical functions, you can compute new variables based on highly complex equations. In this example, however, we will simply compute a new variable that is the difference between the values of two existing variables.

2) The data file demo.sav contains a variable for the respondent's current age and a variable for the number of years at current job. It does not, however, contain a variable for the respondent's age at the time he or she started that job. We can create a new variable that is the computed difference between current age and number of years at current job, which should be the approximate age at which the respondent started that job.  

► From the menus in the Data Editor window choose:

Transform > Compute Variable...

► For Target Variable, enter jobstart.

► Select Age in years [age] in the source variable list and click the arrow button to copy it to the Numeric Expression text box.

► Click the minus (–) button on the calculator pad in the dialog box (or press the minus key on the keyboard).

► Select Years with current employer [employ] and click the arrow button to copy it to the expression.

► Click OK to compute the new variable.

The new variable is displayed in the Data Editor. Since the variable is added to the end of the file, it is displayed in the far right column in Data View and in the last row in Variable View.

3) You can also use predefined functions in expressions. More than 70 built-in functions are available, including:

• Arithmetic functions

• Statistical functions

• Distribution functions

• Logical functions

• Date and time aggregation and extraction functions

• Missing-value functions

• Cross-case functions

• String functions

Using Functions in Expressions

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4) Functions are organized into logically distinct groups, such as a group for arithmetic operations and another for computing statistical metrics. 

5) For convenience, a number of commonly used system variables, such as $TIME (current date and time), are also included in appropriate function groups.

 

6) A brief description of the currently selected function (in this case, SUM) or system variable is displayed in a reserved area in the Compute Variable dialog box.

 

7) To paste a function into an expression:

► Position the cursor in the expression at the point where you want the function to appear.

► Select the appropriate group from the Function group list. The group labeled All provides a listing of all available functions and system variables.

► Double-click the function in the Functions and Special Variables list (or select the function and click the arrow adjacent to the Function group list).

8) The function is inserted into the expression. If you highlight part of the expression and then insert the function, the highlighted portion of the expression is used as the first argument in the function.

9) The function is not complete until you enter the arguments, represented by question marks in the pasted function. The number of question marks indicates the minimum number of arguments required to complete the function.

► Highlight the question mark(s) in the pasted function.

► Enter the arguments. If the arguments are variable names, you can paste them from the variable list.

Using Conditional Expressions

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10) You can use conditional expressions (also called logical expressions) to apply transformations to selected subsets of cases. A conditional expression returns a value of true, false, or missing for each case. If the result of a conditional expression is true, the transformation is applied to that case. If the result is false or missing, the transformation is not applied to the case.

11) To specify a conditional expression:

► Click If in the Compute Variable dialog box. This opens the If Cases dialog box.

► Select Include if case satisfies condition.

► Enter the conditional expression.

12) Most conditional expressions contain at least one relational operator, as in:

age>=21

or

income*3<100

13) In the first example, only cases with a value of 21 or greater for 

Age [age] are selected.

14) In the second example, Household income in thousands [income] multiplied by 3 must be less than 100 for a case to be selected.

15) You can also link two or more conditional expressions using logical operators, as in:

age>=21 | ed>=4

or

income*3<100 & ed=5

16) In the first example, cases that meet either the Age [age] condition or the Level of education [ed] condition are selected.

17) In the second example, both the Household income in thousands [income] and Level of education [ed] conditions must be met for a case to be selected.