Box plots of the frequency of postsurgical pain in patients randomly assigned to receive perioperative fentanyl (gray boxes) or methadone (plain boxes) perioperatively 1 month, 3 months, 6 months, and 12 months after cardiac surgery. The frequency of any pain related to surgery was measured using an ordinal scale, with 0 = less than once per week, 1 = once per week, 2 = twice per week, 3 = daily, and 4 = constantly. Horizontal lines with black diamonds at their center across the boxes represent medians; lower and upper edges of the boxes represent the first and third quartiles; whiskers indicate the data range. The frequencies of postsurgical pain were compared between groups at the various times using the Mann–Whitney U test. The resulting P values are indicated above the data used in the analyses. The criterion for rejection of the null hypothesis was P < 0.01. Sample sizes range from 58 and 46 in the fentanyl and methadone groups, respectively, at 1 month, to 34 and 31, respectively, at 12 months.