The preceding discussion is a broad and somewhat eclectic overview of the human capital link in the relationship between learning and loyalty. At this point, it is useful to summarize the conclusions from this discussion that contribute to a better understanding of interface loyalty. First, it is clear that most human learning proceeds along a learning curve that can be approximated by the Power Law of Practice. This means that with practice, skills are acquired that reduce the time required to complete a task. Second, the human capital model suggests that this reduction in task-completion time has an economic value such that, as practice with a particular interface increases (that is, learning progresses and the time required to complete a task decreases), the cost of using that interface decreases. Third, as the cost of use decreases, the user's preference for the incumbent interface relative to that for alternative interfaces increases to the extent that the acquired skills are non-transferable. While transferable skills may make an entire product category or type of software more attractive, interface loyalty is driven by the acquisition of non-transferable skills. In essence, the human capital model suggests that users "will direct their consumption and search activities in ways that maximize the impact of their accumulated expertise" . This is an important principle in the design of electronic interfaces, whether those interfaces facilitate online shopping, word processing, statistical analysis, or other activities. New users will tend to prefer interfaces with features that are familiar, and to which they can apply skills they have previously acquired. However, our research and the human capital model suggest that, in order to promote the development of long-term user loyalty, it is also important to encourage the development of non-transferable user skills. Following Card et al. , who argue in their third principle of user-interface design that it is imperative that interface designers know their users, we recommend that early in the market research and product development process an effort be made to understand the accumulated human capital of the target user group(s). It is important to consider that, from a user perspective, the cost of a new piece of software or of a purchase at a new online store incorporates not only the price of the product, but also the time cost associated with learning to use the product or to complete the purchase transaction.