PROBIT logistic regression showing the dose–response relationship curve between the mean tidal volume (ml/kg predicted body weight) (A) and mean positive end–expiratory pressure (PEEP) (cm H₂O) (B) used in surgery and the probability of postoperative pulmonary complications. Solid line = mean quadratic term; dashed line = 95% CI. The line represents the quadratic term fitting all the points. The flat line in the PEEP graph suggests that there is neither a positive nor a negative association between a higher level of PEEP and the development of postoperative pulmonary complications.