Table 1. Description of factors tested in the regression.

Level of factor	Factor	Groups	Description, including justification for groups	Source of data	% Missing values
Patient	Patient age	0–2, 3–10, 11–20, 21–30, 31–40, 41–50, 51–60, 61–70, 71–80, 81–90, > 90	Age was grouped because age is sometimes given by a caller who guesses the patient age. 0–2 group was used because some ambulances services reported different non-transport policies for children under 2 years old.	CAD	7.4%
	Patient sex	Female, Male	Only two categories available	CAD	6.2%
	Time of call	Out of Hours, In Hours	Time of call was dichotomised into ‘In hours = 8am-6pm weekdays’ and ‘Out of hours = all other times’. It was grouped because qualitative interviews identified perceptions that the availability of services that facilitated non-transport was better during normal working hours for health services.	CAD	0%
	Source of call	999, 111	In England patients call 999 directly or are passed to 999 after calling the urgent care telephone service NHS 111	CAD	0%
	Type of caller	Patient Health practitioner	All calls from a patient, family, friend or bystander were labelled as ‘patient’. Community nurses and general practitioners can call for an emergency ambulance on behalf of patients and were labelled ‘health practitioner’	CAD	Variable missing for one ambulance service (11.7%)
	Reason for call	Falls, Abdominal Pain, Breathing difficulties, Cardiovascular, Fitting, Injury, Psychiatric, Sick or Unconscious, Other	There are many codes for reason for call and they differ by the two triage software systems used by ambulance services. A small working group of clinicians, experts within the research team, and a research paramedic met to develop common categories from the two triage software systems and identified specific codes where a large proportion were not-transported; all other reasons were classed as ‘other’	CAD	29.4%
	Assessment of urgency	Red 1 & 2 (emergency), Green 1 & 2 (urgent), Green 3 & 4 (low acuity)	Codes such as Red 1 and 2 were grouped together due to small numbers	CAD	1.1%
	Indices of Multiple Deprivation (IMD)	Quintiles Q5 (Least Deprived), Q4, Q3, Q2, Q1 (Most Deprived)	The IMD is the official measure of deprivation of small areas in England, ranking every small area from 1 (most deprived) to over 30,000 (least deprived). It is based on seven aspects of deprivation including income and employment. Quintiles were used because this is a common approach to using IMD in regressions. The variable describes the area from which the call was made.	Census	3.7%
	Urban-rural status	Urban, Rural	The Rural Urban Classification is an official statistic used in the census to distinguish rural and urban areas. There are 4 urban and 6 rural categories. The urban/rural dichotomy was used. Rural areas are outside settlements with more than 10,000 resident population. The variable describes the area from which the call was made.	Census	3.7%
	% population with no central heating	Quintiles	The percentage of the population in the small area from which the call was made that reported having no central heating in the census. Used to represent the quality of housing of a patient.	Census	3.7%
	% population living alone	Quintiles	The percentage of the population in the small area from which the call was made that reported living alone in the census. Used to represent informal support unavailable for patients.	Census	3.7%
	% population with English not as their first language	Quintiles	The percentage of the population in the small area from which the call was made that reported not having English as their first language in the census. Used to represent ethnic groups where communication might affect decision to transport.	Census	3.7%
	% population with severe long term illness	Quintiles	The percentage of the population in the small area from which the call was made that reported having a severe long term illness. Used to represent health status of patients.	Census	3.7%
	Skill-mix	Paramedic, Paramedic extended skills, Other	Each ambulance service uses different labels and codes for the skill-mix of crew attending the scene. A small working group of clinicians, experts within the research team, and a research paramedic met to develop common categories for skill-mix. Each ambulance service was requested to link the labels and codes they used to these common categories, based on the highest skill-mix of attending ambulance crew. This data was not available in CAD but held in another routine dataset by ambulance services and was linked to the CAD data by the ambulance service. Paramedics with extended skills, or advanced paramedics as they are called in England, are defined by the national College of Paramedics as experienced autonomous paramedics with masters degrees in a subject relevant to their practice. ‘Other’ mainly included emergency medical technicians and a small number of doctors and nurses.	Ambulance routine data	Variable missing for one ambulance service and some missing values for all other services (14.4%)
Ambulance service	Workforce configuration	% patients attended by paramedics with extended skills	The patient-level variable on skill-mix was used to create an ambulance service-level variable of the percentage of calls attended by paramedics with extended skills to represent the size of the workforce made up of paramedics with extended skills within each ambulance service	Ambulance routine data	Missing for one service (14.4%)
	Complexity of emergency and urgent care system	Medium, high, low	In the qualitative interviews interviewees described how ambulance crews having to move between areas run by different healthcare commissioners (these are called clinical commissioning groups) reduced the ability to discharge at scene because each area had different services with different referral pathways which ambulance crew needed to know about in order to discharge at scene. Also, although a lead healthcare commissioner worked with the ambulance service to devise a contract for providing non-transport, sometimes individual commissioners from these clinical commissioning groups set up their own contracts with the ambulance service. The CAD system identified the number of clinical commissioning groups covered by each ambulance service to represent the complexity of the external system that an ambulance service had to deal with.	CAD	0%
	Type of triage software	AMPDS NHS Pathways	One ambulance service used two types of software in different geographical regions and was coded as using the software triaging the majority of callers	Ambulance Information Team	0%
	Stability of the organisation	No changes, Significant changes	Staff perceptions of changes occurring to senior management or the effects of external assessments of service quality	Qualitative study	0%
	Organisational motivation for non-transport	No view or mixed views, viewed as opportunity, risk aversion	Staff perceptions of motivation of senior management to undertake non-transport	Qualitative study	O%
	How extended paramedics are used	No view or mixed views, in limited capacity, established and valued	Staff perceptions of whether paramedics with extended skills were used in the ambulance service	Qualitative study	0%
	Fear of retribution	No evidence, Low levels of fear, Evidence of fear	Staff perceptions of level of fear of retribution amongst paramedics if non-transport resulted in adverse events	Qualitative study	0%
	Provision of services in the wider system	Inconsistent views, lacking in provision, good provision	Staff perceptions of availability of services in the wider emergency and urgent care system that facilitated non-transport	Qualitative study	0%
	Connectivity with wider system	Inconsistent views, lacking connectivity, good connectivity	Staff perceptions of how connected an ambulance service was to other services within the wider emergency and urgent care system	Qualitative study	0%
	Commissioners	Worked with some localities only, poor, good	Staff perceptions of the quality of the relationship between the ambulance service and their health care commissioners	Qualitative study	0%
	Telephone advice	Limited use, negative views, enthusiastic senior management	Staff perceptions of provision of this type of non-transport within their ambulance service. Applicable to telephone advice analysis only	Qualitative study	0%
	Cost per call	Medium, low, high	Cost per call was calculated by the National Audit Office by dividing an ambulance service’s urgent and emergency care income by the number of calls presented to its switchboard. It was calculated to represent cost-effectiveness. It was tested in the regression because it was available and varied by ambulance service rather than there being a clear rationale for its potential influence on non-transport. 10 ambulance services grouped into three groups of high, medium and low	National Audit Office	0%
	Cost per face-to-face attendance	Medium, low, high	See explanation for ‘cost per call’. 10 ambulance services grouped into three groups of high, medium and low	National Audit Office	0%
	Staff absence rate	Medium, low	10 ambulance services grouped into three groups of high, medium and low. No service had a higher rate so only two categories were used	National Audit Office	0%
	% frontline staff with extended skills	Medium, high	The percentage of frontline staff with extended skills is similar to the workforce configuration variable above. However, it also includes staff offering telephone advice and does not measure the percentage of incidents attended by different skill-mix. 10 ambulance services grouped into three groups of high, medium and low	National Audit Office	0%
	Income per head of population	Medium, low, high	10 ambulance services grouped into three groups of high, medium and low	National Audit Office	0%