Finally for the multivariable model, the investigators will adjust for confounders, specifically for sex (m/f), history (Hx) of PONV (y/n), Hx of motion sickness (y/n), current smoker (y/n), age (years), expected duration of surgery (min; >60 min), postoperative opioids (y/n), the use of preemptive PONV Therapy (0,1,2 medications), propofol vs. inhalative agents (y/n), and surgery type (3 kinds) from the patient's electronic record. In the event that insufficient event rates are observed, we will collapse the variables into the simplified Apfel score to assess the independent value of fentanyl in predicting PONV.