List of potential confounders for consideration in each of the statistical models
| Variable | Data source |
| Age | PICANet and CMP |
| Sex | PICANet and CMP |
| Ethnicity | HES, PICANet and CMP |
| Pre-existing comorbidities (or a proxy, eg, time in hospital in the previous year) | HES, PICANet and CMP |
| Deprivation (IMD score) | PICANet and CMP (via postcodes or LSOA) |
| Diagnosis | PICANet, CMP and HES |
| Severity of illness (PIM-2/PIM-3 score and individual variables from first contact with transport team, not from referral) | PICANet |
| Ventilated at referral | PICANet |
| Previous admission to intensive care | PICANet and CMP |
| Volume of referring hospital | HES or RCPCH data |
| Interventions prior to arrival of the transport team | PICANet |
| Referral location (A&E, HDU, theatre, ward etc.) | PICANet |
| Season (winter defined as Nov to Feb) | PICANet |
| In hours versus out of hours (to include evenings and weekends) | PICANet |
| Interventions conducted when retrieval team arrives | PICANet |
| Grade of team leader | PICANet |
CMP, Case Mix Programme; HDU, High Dependency Unit; HES, Hospital Episode Statistics; IMD, Index of Multiple Deprivation; LSOA, lower super output area; PICANet, Paediatric Intensive Care Audit Network; PIM, Paediatric Index of Mortality; RCPCH, Royal College of Paediatrics and Child Health.