That may have been a terrible title but I cant think of how to phrase it.
For my study participants were asked to complete a 21 item scale, and were then asked to repeat the same scale later on. How I would I differentiate between these and measure them separately within SPSS?
Hello Dominic,
Sorry if I don't fully understand your query, so the reply may seem a bit general.
1. Enter both sets of responses (e.g., pre_01, pre_02, ..., pre_21, post_01, post_02...post_21) in the same record as variables (42 in all) for each respondent.
2. Create scale totals by summing the respective items from a given occasion (e.g., compute pre_total = pre_01 + pre_02 + ... + pre+21, or whatever assortments make sense given your scale).
3. Now you can access and directly compare the two scores (or sets of scores), appropriately matched by participant.
Good luck with your work.
I agree with David Morse that the key goal should be to create two scales -- one for the first set of items and the other for the second set. In addition, you should assess the reliability of these scales, beginning with coefficient alpha. But alpha cannot tell you whether all your items form a single scale, so for that you should use exploratory factor analysis. For the factor analysis, the default values in SPSS are typically not appropriate for assessing scales; instead I recommend principal axis or maximum likelihood extraction and correlated factors (oblique rotation).
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