A psychologist working in the county jail is interested in the effectiveness of mindfulness on stress management among jail inmates. The mindfulness intervention consists of six 90 minute sessions. Patients are administered the Perceived Stress Scale (PSS) at the beginning of their first session, and then re-administered the PSS at the end of treatment (session 6), and again 30 days after treatment has ended. What statistical test should be used to measure the effectiveness of this group/intervention? Identify the variables (and levels, as applicable), and its scale of measurement.