Randomization and allocation concealment
Participants were randomly assigned to one of two groups. Random tables were generated by SPSS 20.0. One hundred and twenty sealed envelopes were prepared by a statistician who did not participate in the study. The study was performed with neither patients’ nor observers’ awareness of the group to which each patient belonged. To assure concealment of allocation, numbers were kept in sealed and opaque envelopes, which were opened by an anaesthesiologist who was not involved in the study.