(b) When making a decision on whether to further process two joint products, the costs incurred to bring those products to the separation point have already been incurred and so are sunk costs. In addition, as the products are not separately identifiable before the separation point, any apportionment of joint costs to joint products is necessarily arbitrary. The only relevant costs in a further processing decision are those which are incurred after the separation point.

Joint processing costs are, however, relevant to assessing whether the whole process is viable, as the total costs to ready the products for sale need to be compared to the revenues earned to determine if the overall process generates a profit. In addition, the joint costs are avoidable if the joint process is closed.