Table 1. Datasets for linkage.

Data source	Description	Summary of data items for extraction
National Neonatal Research Database (NNRD) [22]:	Contains care data for all babies admitted to NHS neonatal units across UK since 2007	Baby’s demographics, neonatal clinical care, diagnoses, outcomes up to neonatal discharge, outcomes at two-year review.
Office for National Statistics (ONS) [23]	Collects data regarding date and cause of death for all registered deaths in England and Wales.	Death registration, including date of death and cause of death.
Hospital Episode for Statistics (HES) [24]	Contains data on hospital admissions to NHS hospitals, outpatient appointments, and attendances at accident and emergency departments across England. Collects data related to patient demographics, diagnoses and clinical care.	Demographics, clinical care and diagnoses.
Secure Anonymised Information Linkage (SAIL) databank [28]	Contains annual district birth and death extracts, congenital anomaly register, education data for Wales, critical care dataset, patient episode database Wales, national community child health database, Welsh demographic service dataset, Welsh longitudinal general practice dataset, Wales results reporting service, maternal indicators dataset.	Demographics, clinical care, diagnoses including congenital anomalies, education, births and deaths, primary and secondary care use, community child health resource use and maternity data.
Paediatric Intensive Care Audit Network (PICANet) [25]	Contains details of the treatment of all critically ill children in paediatric intensive care units (PICU) across the UK. Data includes: demographics; clinical diagnoses; treatment received in PICU and outcomes at discharge.	Demographics, clinical care, diagnoses and outcomes
Mental Health Services Dataset (MHSDS) [26]	The MHSDS contains individual level data for all children accessing mental health care across the community, outpatient, and inpatient settings in England.	Constant supervision and care required due to disability indicator, looked after child indicator, child protection plan indication code, care professional service or team type association (mental health), disability code.
The National Pupil Database (NPD) [27]	Managed and controlled by the Department for Education and held in the ONS Safe Haven. The NPD contains detailed information on the educational attainment, SEN, and attendance of children at state schools across England between the ages of 5–18 years.	The School Census (including alternative provision census and pupil referral unit census, Children in Need, Children Looked After, School Absences, School Exclusions); Early Years Foundation Stage Profile (EYFSP) data at age 5, Phonics data, Attainment, Good for Development (reaching expected attainment), Key Stage 1 data, Key Stage 2 data with attainment scores, Education, health and Care plan (EHC), Special Educational Needs include: primary and secondary SEN type, Disability Access Funding (For 3–4 year olds), eligibility for free school meals.