Severity of pain for those patients with and without chronic pain related to thoracic surgery at 6 months after surgery. Note that those patients with zero pain scores were included in this plot. When analyzed longitudinally, both the time effect (P < 0.0001) and the presence of pain at 6 months (P < 0.0001) were significant, but not the interaction (P = 0.62) term. Note that, one patient (¥) was sedated during the first 3 days. Some (5 patients at 3 months and 8 patients at 6 months) patients completed phone interviews at 3 and 6 months but did not return the mail including the numerical rating scale (NRS). One-sided error bars represent SD. POD = postoperative day; pre-op = preoperative. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001, ****P ≤ 0.0001.