Changes to the National Child Measurement Programme 2019-20 publication content and disclosure control methodology

In scope

The new methodology will be applied by the Analytical Services - Population Health, Clinical Audit and Specialist Care analytical team at NHS Digital in producing and disseminating outputs using the National Child Measurement Programme (NCMP) dataset from October 2020.

This will include

• the National Child Measurement Programme, England 2019/20 School Year release due to be published 29 October 2020
• tabulations and bespoke table requests produced after 29 October 2020
• responses to Parliamentary Questions, Freedom of Information requests and media queries from 29 October 2020

When producing analysis we need to balance accuracy and timeliness of publication with disclosure control to reduce the risk of identifying individuals from the outputs.

The current NCMP disclosure control rules use a combination of suppression any values between 1 and 5 with a “*”, then applying secondary suppression on the outputs to ensure that it is not possible to recalculate any of the suppressed values in the outputs.

This is a manual process for each output, which takes considerable time, has a risk of error, and can produce inconsistent outputs.

A disclosure control method that can be automated will be more efficient and provide greater security, particularly when it is used across separate statistical releases. It will also help us to provide more timely data to customers, particularly at sub-national geographies.

For the NCMP we will be implementing the same disclosure control rules as those currently used on the NHS Digital secondary care datasets.

Full details of the secondary care methodology is available.

Please refer to the above paper for detailed information on the impact of using rounded data in the calculations of percentages and rates.

No disclosure control required for small numbers.

The following steps will be applied to reduce the risk of identifying individuals from small numbers.

1) If the national total is between 1 and 7 (inclusive), no sub-national breakdown will be displayed.

2) If the national total is greater than or equal to 8

a. Sub-national counts between 1 and 7 (inclusive) will be displayed as ’*’.

b. Zeroes will be unchanged.

c. All other counts will be rounded to the nearest 5.

Note that row/column totals will be calculated and then rounded, which means that the total of rounded values may differ from the rounded total.

For calculations such as percentages and rates it is often possible to work out the numerator and denominator because only one possible pair of values would give the percentage or rate being displayed, especially where the percentage is presented to a higher degree of precision than 2 decimal places. Therefore, disclosure control also needs to be applied to simple calculations such as percentages, rates, mean, median or mode.

No disclosure control is required.

1) All simple calculations are undertaken using rounded data. For example, the numerator and denominator for a percentage will be rounded to the nearest 5.

2) Where the count of data in any part of the calculation is between 1 and 7 inclusive, no calculation is made and a ‘*’ is displayed. For example if the numerator of a percentage is between 1 and 7 a ‘*’ will be displayed in the outputs.

3) Where the numerator is zero, the percentage will be 0%.

4) Where the count of data in any step in the calculation is greater than or equal to 8, the calculated value is displayed.

For complex calculations such as standardised rates, confidence intervals and regression models, no disclosure control is required.

Note the analytical team at NHS Digital producing the NCMP outputs will be applying and additional rules to the calculation and presentation of confidence intervals in the outputs they produce. Where the prevalence data has been suppressed as the numerator is between 1 and 7, the confidence interval will also be suppressed from publication. This does not form a core requirement of the disclosure control measures but is aimed at producing consistent and useful results from the NCMP dataset. 

Disclosure control only needs to be applied to values relating to individuals. No rounding or suppression is required for values not relating to individuals such as a count of schools or local authorities.

In previous NCMP years, the measurement of children and submission of the national dataset by local authorities happens throughout the full school year between September and July.

In response to the covid-19 pandemic, all schools were closed on 20th March 2020 and the national child measurement programme was deprioritised across England. The NCMP IT system remained available so that local authorities could submit any data they had collected earlier in the year, but it was not mandatory for them to do this. This year there is no expectation that local authorities will meet the normal minimum participation rate of 90% in the data they submit to NHS Digital (as set out in the NCMP operational guidance 20191 ). Therefore, the NCMP 2019/20 will only include data for children who were measured before schools closed in March and will be smaller than previous years.

To reflect this the content of the national report 2019/20 will differ to previous years:

The report will have a much larger data quality section. Detailed work will be undertaken to assess the impact of a partial year of data on reporting. Data quality reporting will include commentary, data tables and charts where appropriate.

A reduced number of national data tables will be included in the report, prioritising those where the data is most complete.

This year a smaller number of local authority data tables will be produced. It is expected that the school closures will have had a greater impact on certain local authorities (LAs) that had planned to measure children between March and July than those that has measured children earlier in the year. Therefore, we propose to divide LAs into 3 categories in reporting

• LAs where the number of children measured is very low will be unreliable and suppressed from publication
• LAs with partial data will be released but caveats and footnotes added to describe the limitations of the data
• LAs where most children have been measured will be released as normal

Products that will not be produced this year

The anonymised NCMP csv file and guidance document will not be produced for 2019/20, with the resources usually used to produce it being refocussed on the additional data quality work detailed above. A further review of this file will be undertaken in 2020/21.

We welcome any questions, comments or feedback relating to the new disclosure control method and other changes detailed in this paper.

Please quote “Feedback on changes to the NCMP publication” in the subject title of any correspondence via the contact methods listed below.

Email: [email protected]

Telephone: 0300 303 5678

NHS Digital 1 Trevelyan Square Boar Lane Leeds LS1 6AE.