In statistics, the term "official data" denotes data collected in censuses and statistical surveys by National Statistics Institutes (NSIs), as well as administrative and registration records collected by government departments and local authorities. They are used to produce "official statistics" for the purpose of making policy decisions. The public availability of these large official data sets is considered a new challenge for the data mining community. Very few applications of data mining techniques in the discovery of new models or patterns in official data sets have been reported.