The Requirements Entropy Framework in Systems Engineering

This paper introduces a requirements entropy framework (REF) for measuring requirements trends and estimating engineering effort during system development. The REF treats the requirements engineering process as an open system in which the total number of requirements R transition from initial states of high requirements entropy HR, disorder and uncertainty toward the desired end state of HR min as R increase in quality. The cumulative requirements quality Q reflects the meaning of the requirements information in the context of the SE problem. The distribution of R among N discrete quality levels is determined by the number of quality attributes accumulated by R at any given time in the process. The number of possibilities P reflects the uncertainty of the requirements information relative to HR min . The HR is measured or estimated using R, N and P by extending principles of information theory and statistical mechanics to the requirements engineering process. The requirements information I increases as HR and uncertainty decrease, and ΔI is the additional information necessary to achieve the desired state from the perspective of the receiver. The HR may increase, decrease or remain steady depending on the degree to which additions, deletions and revisions impact the distribution of R among the quality levels. Current requirements volatility metrics generally treat additions, deletions and revisions the same and simply measure the quantity of these changes over time. The REF measures the quantity of requirements changes over time, distinguishes between their positive and negative effects in terms of Q,HR, and ΔI, and forecasts when a specified desired state of requirements quality will be reached, enabling more accurate assessment of the status and progress of the engineering effort. Results from random variable simulations suggest the REF is an improved leading indicator of requirements trends that can be readily combined with current methods. The additional engineering effort ΔE needed to transition R from their current state to the desired state can also be estimated. Simulation results are compared with measured engineering effort data for Department of Defense programs, and the results suggest the REF is a promising new method for estimating engineering effort for a wide range of system development programs.