Disclosure Control of Microdata

National statistical offices meet an increasing demand for the dissemination of microdata. This trend can be at variance with the care taken not to disclose data about individuals. Therefore, the risk of disclosure of each microdata set to be released should be assessed. If this risk is too high, measures have to be taken to protect the data set. This article describes the disclosure problem, and explains why it is a real problem. Using the concept of uniqueness, some theory is developed that can help establish the risk of identification. It turns out that useful microdata sets can only be released if some of the disclosure risks are dealt with by legal arrangements, rather than by restrictions on the data to be released.