Many manufacturers find the heaviest of these costs to be the opportunity cost of the personnel. Manufacturers often have no one available to be diverted from the core activity of manufacturing, and as they do not know the market for expertise in distribution, these firms find it difficult to hire qualified personnel to meet service output demands economically. The top management of the vertically integrated firm is responsible for ensuring distribution and often finds that it does not have sufficient managerial resources to give attention to the distribution responsibility. (Of course, this and every other argument can be reversed for downstream channel members integrating upstream-this is a useful mental exercise.)