http://researchbriefings.files.parliament.uk/documents/CBP-7840/CBP-7840.pdf
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/766465/The-UKs-future-skills-based-immigration-system-print-ready.pdf
https://www.publications.parliament.uk/pa/cm201617/cmselect/cmwomeq/799/799.pdf
http://www.bertelsmann-stiftung.de/fileadmin/files/Projekte/Migration_fair_gestalten/IB_Study_MixedMigration2017_ENG.pdf