This summary presents the main findings from a data mapping and analysis study commissioned by the Local Government Association (LGA) from the National Foundation for Educational Research (NFER) as part of a two-year development and research programme, Narrowing the Gap. This programme seeks to make a significant difference in "narrowing the gap" in Every Child Matters (ECM) outcomes between "vulnerable" children (aged 3 to 14) and other children. These outcomes are to be healthy, to stay safe, to enjoy and achieve, to make a positive contribution and to achieve economic well-being. The data study, carried out in September and October 2007, sought to identify what useful and comparable data was (and was not) readily available on each of the outcomes for vulnerable groups. The collection of individual child-level data on PLASC and in some of the longitudinal large-scale surveys facilitated the identification of changes over time against some of the ECM outcomes. The ways in which data is currently collected and collated, however, and the different ways in which groups are defined mean that it is not possible, in most cases, to ascertain whether or not the gaps are narrowing, or widening or have stayed the same in recent years. The report concludes that physically, operationally and ethically, there are likely to be problems in constructing one single database for all young people. Comprehensive, child-level datasets provide the richest source of illuminative data, while representative surveys can be a source of enlightening insights. In using and matching these datasets there is clearly a need to consider the ways in which best to approach the use of sensitive data, particularly in relation to data protection and child protection issues. Without a more strategic approach to the collection and analysis of data, preferably adopting a multivariate approach, it is anticipated that significant difficulties in identifying gaps and monitoring progress towards ECM will remain. This is identified as a challenge for the wider project on Narrowing the Gap that merits further consideration. (Contains 2 figures and 2 tables.) [This report was written with with Simon Rutt, Lesley Kendall, and Palak Mehta. For associated Research Evidence Review, see ED502360.]