Intruder Testing on the 2011 UK Census: Providing Practical Evidence for Disclosure Protection

Reflecting the international movement to widen access to public datasets, NSIs need to provide solid evidence not to publish data they believe may represent a disclosure risk. Under complementary legislation such as the UK Freedom of Information Act, theoretical risk assessments alone are often not enough to satisfy a court of law. The 2011 UK Census is a high-profile and rich data resource and outputs include detailed multivariate statistics down to local area level presenting potentially abundant disclosure risk. There is strong user demand for small cell counts unobscured by disclosure control. This paper uses the 2011 UK Census as an example to demonstrate the use of intruder testing in obtaining specific and practical evidence for disclosure protection. Using a structured approach with real people as intruders, original insight is gained into notions of disclosure and perceptions of how the data are protected.