A Key File is a file which results from anonymizing activities. It contains personal data elements (e.g.) a name) and a code with which the occurrences of that name is replaced within the transformed data files. With key files data can be re-identified. Hence they should never be stored on the same locations as the research data and accessibility should be minimized. N.B. as long as a Key File exists a data set is not anonymized, but pseudonymized.