Horizontal integration occurs when an activity results in end-products with different characteristics. This could theoretically be interpreted as activities carried out simultaneously using the same factors of production, in which case it would often be impossible to separate such activities statistically into different processes, assign them to different units or generally provide separate data for them, nor would rules relying on allocation of value added or similar measures be applicable. Alternative indicators, such as gross output, might sometimes be applicable, but there is no general rule for identifying the single activity that best represents the mix included in this horizontal integration. Since patterns of horizontal integration have been considered in the preparation of the classification, in many cases commonly integrated activities are included in the same class of activity even though their outputs have quite different characters.