20 pаtients with chrоnic аrthritis pаin were given a new treatment tо alleviate the pain. The same patients were tested at 3 mоnths, 6 months, and 9 months after first receiving treatment. At each assessment point, patients indicated level of pain from 0 (no pain) to 10 (severe pain). What statistical test would be used to examine whether the new treatment was effective at reducing arthritis pain over time?