Defining children as overweight or obese is a complex process, given that children of different ages and sexes grow and develop at different rates.

This means that a different method is used for children than for adults.

BMI is calculated by dividing their weight (in kilograms) by the square of their height (in metres), and for children, this is then compared to a reference sample of measurements gathered in 1990, which takes age and sex into account.

Further information is available in a guide to classifying body mass index in children produced by Public Health England (PHE).

Before the programme starts each school year, local authorities write to the parents and carers of all children eligible for measurement to inform them of the programme.

Parents or carers can choose to withdraw their child from the process by responding to the letter.

Local authorities are asked to collect data on Reception Year and Year 6 children's height and weight from all state maintained schools within their area.

Prior to a legislation change in 2013, this role was fulfilled by NHS Primary Care Trusts.

The data is submitted to NHS Digital, and all of the returns are collated and validated centrally.

Public Health England (PHE) provide operational guidance to local authorities and schools on how to undertake the exercise, which states:

• local authorities will take steps to ensure that parents receive a letter explaining the purpose of the programme, and provide them with the opportunity to withdraw their child from it
• if local authorities are routinely feeding back results to parents, they aim to do this within 6 weeks of measurement
• the privacy and dignity of the child must be safeguarded at all times, and the measurement is to be done sensitively in a private setting
• individual children's results will not be shared with school staff or other pupils, and suppression and disclosure controls will be implemented when the data set and publication is released to ensure that individual children cannot be identified

NHS Digital use the data to produce National Statistics Reports showing trends at national level, differences between groups of pupils and the latest local authority level data.

The data is also shared with the Public Health England Obesity Risk Factors Intelligence team (PHE Obesity RFI) but in a form that in no way could lead to an individual child being identified. PHE present the data in an online tool that enables the user to examine patterns and trends at local authority level.

No information is ever published by NHS Digital or PHE in a way that could lead to individual children being identified.

The information may also be linked with previous height and weight measurements and other data, such as dental survey or hospital episode statistics, or provided to other organisations, such as universities for research into health and social care. Any linkage or sharing with other organisations is subject to approval from an independent group of experts.

NHS Digital has shared the data with PHE for linkage to their dental survey data. The data shared with PHE was in a form that no child be identified. A link to the report from this study will be included here when it becomes available.

NHS Digital has also shared the data with the University of Essex to assess the impact of the Universal Free School Meals programme for children in Key Stage 1 on child weight. A report will be published and a link made available here in due course. The data shared did not include any child level data.