A major challenge when contextualising admissions to university, or recruitment for jobs, is access to high quality information on a young person's background, to identify those who should benefit. Granular and verifiable information about prospective students' socio-economic background is, in practice, limited. As a consequence, universities and employers often need to use 'proxy' measures, for example looking at the local area someone grew up in based on their home postcode. But little is known about how well these measures capture individual-level socio-economic status. This report uses data from the Millennium Cohort Study to look at how various proxies for family background correlate with long-run family income, based on data for over 7,000 children.