Table 2. Potential confounders or effect modifiers within the linked data sets available for analysis in addition to the outcome variables (*matched variable).

NNRD (cohort 1 and 2 only)	PDS/HES	NPD and social care data
Year of birth*Month/season of birthSex*Gestation*Birth weight Z-score*Multiplicity*Antenatal corticosteroids*Antenatal magnesium sulfate*Receiving breastmilk at dischargeMode of deliveryNeonatal unit characteristicsAdmission neonatal unit levelAdmitting operational delivery networkDischarging neonatal unit levelDischarging operational delivery networkNumber of different neonatal unit admissionsMaternal characteristicsMaternal smoking statusIndex of multiple deprivationMaternal ethnicityMaternal ageNeonatal multimorbidityNecrotising enterocolitis (conservatively or surgically treated)Spontaneous intestinal perforationRetinopathy of prematurity (treated)Bronchopulmonary dysplasiaCulture positive sepsisMajor surgeryPatent ductus arteriosus (treated)More than one brain injuryTreatment variablesDuration of ventilationReceipt of surfactantPostnatal corticosteroid treatmentFeeding regimes (feeding routes, types and durations)Therapeutic hypothermia (for infants with HIE)	Season of birthYear of birth*Sex*Gestation*Birth weight*Multiplicity*Maternal ethnicityChild ethnicityIndex of multiple deprivationHospital of birth	School attendance (and exclusions)Child in needLooked after childEligibility for free school mealsFirst languageChild ethnicityQuintile of index of multiple deprivation

HESHospital Episode StatisticsHIEHypoxic Ischaemic EncephalopathyNNRDNational Neonatal Research DatabaseNPDNational Pupil DatabasePDSPersonal Demographic Service