This dimension covers, with respect to the statistics, their proximity between an estimate and the unknown true value.
Full details about validation are provided in NHS Digital's validation document.
A subset of these validations have been undertaken during the production of these provisional data, details are summarised below.
Local Authorities enter data into the NCMP system which validates each data item at the point of data entry. Invalid data items (e.g. incorrect ethnicity codes) and missing mandatory data items are rejected and unexpected data items (e.g. “extreme” heights) have warning flags added.
During the collection the NCMP system provides each Local Authority with real time data quality indicators, based on the data they have entered, for monitoring and to ensure the early resolution of any issues.
Prior to taking the extract in May, Local Authorities were given an opportunity to review the data they had submitted to the system, enter any measurements for children they had already undertaken, and review and fix any warning messages in the records. This was an optional request, and only those Local Authorities with capacity reviewed their data.
A summary of these data quality measures at a national level, as of 23 May 2022, is published in Table A3 alongside this provisional report.
In recognition of the effect of natural year to year variation, confidence intervals are included around the prevalence estimates in the tables and these should be considered when interpreting results. A confidence interval gives an indication of the sampling error around the estimate calculated and takes into consideration the sample sizes and the degree of variation in the data. They are used to determine whether any differences in prevalence figures are likely to be real or due to natural variation.
The sample sizes and participation rates for NCMP are usually large (over one million children measured and a participation rate exceeding 90 per cent for the 2008/09 to 2018/19 collections), so the 95 per cent confidence intervals for prevalence estimates at national level are very narrow. The 2019/20 and 2020/21 collections yielded fewer measurements so have wider, albeit still narrow, confidence intervals (see Table 2 of the report tables). This indicates a small margin of potential error. The comparisons that feature in this report have all been tested at a 95 per cent significance level. Where two figures are described as being different (e.g. higher/lower or increase/decrease etc.) the result of the test has determined a statistically significant difference. Further details are provided in appendix D of the publication.
As the data are based on a sample (rather than a census) of pupils, the estimates are subject to sampling error. Appendix C details how to calculate sampling errors for this survey, and the excel tables include true standard errors and design effects calculated for key survey estimates.
In general, attention is drawn to differences between estimates only when they are significant at the 95% confidence level, thus indicating that there is less than 5% probability that the observed difference could be due to random sampling variation when no difference occurred in the population from which the sample is drawn.
The limitations of the survey estimates are discussed in appendix B.