Observed and mean predicted in-hospital mortality differences between arterial oxygen tension (PaO
) groups. The difference in mean predicted risk of death was significantly different among the groups (P <0.001), it being highest in the hypoxemia group and lowest in the hyperoxemia group. Predicted risk of death matched observed mortality very well within the quartiles with R
2 values between 0.097 and 0.746. The predicted probability of death was calculated using the following variables: acute physiology and chronic health evaluation II (APACHE II) index independent of oxygenation (AP2no-ox), admission year (before or after 2007), emergency operation, intracranial pressure monitoring, controlled hypothermia and platelet count.