String variables in PASW (SPSS) are those that contain letters e.g. participant's names or open-ended responses to a survey. To set a variable as 'string' you will need to change it's 'type'. To do so, within the Variable View, click on the grey box within the 'Type' column for the variable. Change the type to 'String' in the dialog box that appears and then click OK. Note that numerical operations cannot be performed on string variables.