The principal activity is the activity which contributes most to the total value added of the unit under consideration. Ideally, the principal activity of the unit should be determined with reference to the value added to the goods and services produced, by applying the top-down method. The top-down method follows a hierarchical principle: the classification of the unit at the lowest level of the classification must be consistent with the classification of the unit at the higher levels. The principal activity so identified does not necessarily account for 50% or more of the entity's total value added. In case that the value added is not known, substitute criteria can be used as proxies to obtain the best approximation possible compared to the result which would have been obtained on the basis of value added data.