What are the requirements or characteristics of a questionnaire (e.g. test-retest reliability, internal consistency) so that it is appropriate to detect (a) differences between groups (e.g. at a clinical trial) and (b) differences of individual patients (e.g. before and after an intervention) in clinic? I use a 5-point Likert scale.
I also recommend this complete and helpful paper: Cicchetti, D. V. (1994). Guidelines, criteria, and rules of thumb for evaluating normed and standardized assessment instruments in psychology. Psychological Assessment, 6, 284-290.
For to detect differences between groups:
1) you should perform an exploratory or a confirmatory factor analysis
2) check the reliability (ex, calculate Cronbach’s alpha)
3) use a paired sample t-test for check pre and post-test differences
4) use ANOVA for checking group differences
good luck with your research!
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