Randomization and blinding
BLIND
A biostatistician who is unaware of treatments and patient follow-up, generates the random numbers using the SAS software (SAS Institute, USA) in a ratio of 1:1, named group A and group B. The randomization sequence is stored online until the end of the study (Only the biostatistician was aware of the randomization sequence and did not participate in the conduct of the study or patient follow-up. The responsible anesthesiologist was aware of the drugs being used, oxycodone or tramadol, but was not involved in patient assignment, follow-up, or data analysis. The responsible anesthesiologist is aware of whether oxycodone or tramadol is used for the patients, however, he or she is not involved in the patient assignment, follow-up, and data analysis. The study drugs are prepared by the responsible anesthesiologist. The study drugs were prepared by the responsible anesthesiologist, diluted to the same volume with normal saline, and placed in similar PCIA devices without labeling. Throughout the study, all study personnel, patients, parents, and everyone except the responsible anesthesiologist, remained blind to the patients’ allocations.In an emergency, each center’s principal investigator can request the treatment allocation unmasking. The case should be documented and analyzed to evaluate any association with the treatment.