Fig. 1. A total of 327,650 records of patients undergoing anesthesia were identified in Danish Anesthesia Database. Excluding records of anesthesia other than patients undergoing general or combined anesthesia and primarily scheduled for tracheal intubation, the cohort included 129,925 records. Registered intubations were excluded as explained. The subgroup of 13,135 records representing the penultimate intubations of patients intubated more than once were merged to the corresponding last intubation for the specific patient, and hereby confirmed information according to the covariate previous difficult intubation was created. Thus, 78,162 patients were only intubated once, and 13,135 patients were intubated two or more times; information was missing for 35 patients.