Despite the endeavour made to define the classes of ISIC, or the most detailed level of any other national activity classification, in such a way that their activities should be confined to the range of activities described in a single category of ISIC and to the one geographic area in the relevant geographic classification used, some of the activities of establishments, or similar units, which are classified to a given class will be characteristic of other classes of the classification. In compiling data classified according to kind of economic activity, it will therefore be valuable to compute measures of homogeneity in respect of kinds of activity for the units falling into the various categories of the scheme of classification. The two most important ratios to assess the homogeneity of the various categories are the specialisation ratio and the coverage ratio.