4. Patients < 4 y of age were excluded to improve phenotype reliability. Due to a higher empirical risk of postoperative obstruction following T&A, children < 4 y of age are admitted for overnight observation. The titration of morphine is more difficult in these patients as clinicians generally follow more restrictive dosing regimens and the total morphine dose is drawn out over an extended period. Furthermore, the self-report Faces Pain Scale - Revised (FPS-R) has been validated for children ≥ 4 y and will be used to corroborate the morphine response phenotypes that are primarily based on morphine doses administered to achieve low Face, Legs, Activity, Cry, Consolability (FLACC) scores. The latter behavioral scoring is required for early morphine administration as children and adolescents emerging from anesthesia may have difficulty expressing themselves in the first 30 min. In addition, it is important for our statistical analysis to have a sequential scoring system over time that is applicable to all ages. Finally, the incidence of emergence agitation/delirium, a clinical phenomenon that shares some features of pain, but becomes distinct over 15-30 min, is higher in younger patients. (Sikich, 2004; Vlajkovic, 2007) By excluding children < 4 y we decrease the proportion of subjects whose FLACC scores may be high on the basis of emergence agitation/delirium.