Table 1

 Definitions of terms and variables examined

ATS, Australasian Triage Scale.
Total ED overcrowdingOvercrowding refers to the situation where ED function is impeded, primarily because the number of patients waiting to be seen, undergoing assessment and treatment, or waiting for departure exceeds the physical or staffing capacity of the ED.1 This includes patients in cubicles, corridors, and waiting rooms
Access blockAn admitted patient who spends more than 8 hours in the ED1
AdmissionA patient who underwent the formal electronic hospital admission process
Low acuity patientA patient with a problem who, if seen by a general practitioner, would not have presented to an ED16
ED capacityThe number of ED treatment areas, excluding observation wards and corridor spaces
Total ED occupancyProportion of available cubicle hours per month occupied by patients
Access block ED occupancyProportion of available cubicle hours per month occupied by access blocked patients
Australasian triage scoreThe ATS is a 1–5 urgency rating given to all patients presenting to the ED. The Australasian College for Emergency Medicine (ACEM) defines recommended waiting times for each category. ATS 1 patients have life threatening critical illness that should be seen by a doctor immediately, whereas ATS 5 patients are non-urgent patients who can wait up to 2 hours for medical attention
Patients seen within thresholdThe percentage of patients who were seen within their recommended time as defined by the ACEM. This is a good measure of ED performance; a poorly performing ED will be unable to see patients in a timely manner
ED performanceThe performance of an ED is regarded to be how many patients were seen within their recommended waiting time. A department is high performing if it sees most of its patients within recommended times, while a poorly performing department has prolonged waiting times.
Total diversion timeThe total number of minutes the hospitals were on ambulance diversion
Triple diversion requestsThe number of times all three central hospitals requested ambulance diversion at the same time. When this occurs, all diversion requests are cancelled, and ambulances are directed to the nearest hospital or at the discretion of the ambulance shift co-ordinator.
Sea surface temperatureAverage monthly sea surface temperature adjacent to Perth as reported by the Commonwealth Scientific and Industrial Research Organisation