Other sample characteristics

Measures of potential confounding variables were included as detailed elsewhere.²⁸ Briefly, age was calculated to the nearest week using the date of birth, either imported from Medicare Australia’s database at the time of LSAC enrolment (child) or self-reported (parent), and date of assessment. Sex was self-reported.

Body mass index (BMI) was calculated as kg/m², using measured height and weight.²⁸ For children, age-adjusted and sex-adjusted BMI z-scores were calculated using the US Centers for Disease Control and Prevention growth reference charts.³²

Brachial blood pressure was measured using the SphygmoCor XCEL (AtCor Medical Pty, West Ryde, NSW, Australia), following 7 min rest. Mean arterial blood pressure was calculated from the systolic and diastolic measures.

In Australia, Socio-Economic Indexes for Areas provide standardised scores for socio-economic position by geographical area (postcode of family domicile) compiled from 2011 Australian census data. We used the Socio-Economic Indexes for Areas index of Relative Socio-economic Disadvantage (Disadvantage Index) which numerically summarises the social and economic conditions of Australian neighbourhoods (national mean of 1000 and SD of 100, where higher values represent less disadvantage).³³ Parents reported child and parent medical histories via iPad-administered questionnaires at the assessment centre.

Child and parent participants who attended the CheckPoint assessment centres also completed a visual acuity assessment (not conducted in home visits). As part of this assessment, they were asked if they ‘usually wear glasses or contact lenses’. Staff members recorded their response as yes or no; the strength of prescription was not captured.