The ED occupancy rate, defined as the ratio of the total number of ED patients to the number of licensed treatment bays per hour, will be used as a measure of ED crowding. The numerator includes all patients in the ED at any point during each study hour, regardless of ED location (including in the waiting room, boarding, hallway location). The denominator, constant per study hour, includes the total number of licensed treatment bays as defined according to the ED's original blueprint but excludes hallway locations. For each hour of the day, the ED occupancy rate will be derived from the ED information system. 104 weeks are divided into 13-week seasonal blocks and nursing shifts starting 23:00, 07:00 and 15:00. The mean ED occupancy rate is calculated for each shift as the sum of the occupancy rates within that shift divided by 8. The top quartile of all mean ED occupancy rates during the cohort period is considered as crowded. In order to match both 'crowded 'and 'not crowded' cohorts, the following data elements are extracted from the hospital information system for each registered patient: demographic data (age, sex, co-morbidities); referral source, surgical or medical admission; time of arrival (including season, month of year, day of week, time of day); triage category; ED and total hospital length of stay; and final admission diagnosis.