National Statistical Institutes (NSIs) often publish business statistics for a number of economic industries. These industries are determined by a classification of economic activity. In European countries, since 2008, the NACE-code rev. 2 classification is used (Eurostat, 2008). Determining a single NACE code is not an easy task, since statistical units often have multiple activities, some of which are ancillary (such as holding activities). Eurostat (2008, chapter 3) provides rules on how to derive a single NACE code for a statistical unit given certain input information, such as the set of economic activities and their relative importance. We define the true NACE code of a statistical unit as the code that is obtained when these rules are correctly applied, using error-free input data.