For this final link to hold, the relationship must be profitable. Striving to maintain the loyalty of unprofitable customers is not a viable business model. That is why it is important for marketers to assess the profitability of each of its clients (or types of clients), and terminate those relationships that are not profitable. In order to do this, each customer's "relationship costs" are compared to their "relationship revenue". A useful calculation for this is the patronage concentration ratio. This calculation is hindered by the difficulty in allocating costs to individual relationships and the ambiguity regarding relationship cost drivers.