Time is Not Money: The Case for Multi-dimensional Accounting in Value-based Software Engineering

"Time is money," or so goes the old saying. Perhaps, influenced by this aphorism, some strategies for incorporating costs in the analysis of software design express all costs in currency units for reasons of simplicity and tractability. Indeed, in theoretical economics all costs can, in principle, be expressed in dollars. Software engineering problems, however, often present situations in which converting all costs to a common currency is problematical. In this paper we pinpoint some of these situations and the underlying causes of the problems, and we argue that it is often better to treat costs as a multidimensional value, with dimensions corresponding to distinct types of resources. We go on to highlight the differences among cost dimensions that need to be considered when developing cost-benefit analyses, and we suggest mechanisms for mediating among heterogeneous cost dimensions. ERRATUM: On page 3, in the list just above Table 2 and in Table 2, "integer" should read "interval."