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Custom software development projects under product specification uncertainty can be subject to wide variations in performance (duration, cost, and quality) depending on how the uncertainty is resolved. A prototyping strategy has been chosen in many cases to mitigate specification risk and improve performance. The study reported here sought to illuminate the case where duration was the highest priority project constraint, a feature that has often called for a concurrent development process. Unlike concurrent engineering, however, the process in this paper was a sequential, three-phase approach including an optional, up-front prototyping phase, a nominal-duration construction phase, and a variable length rework phase that grew with the arrival of specification modifications. The source of uncertainty was the modification arrival time; the management control point was the amount of time spent engaged in prototyping activities. Results showed that in situations where the modification arrival rate was sufficiently faster during prototyping than during construction, a minimal-duration choice was available. The model also returned the solution to perform no prototyping in cases where arrival rates were nearly equivalent between phases, or when the rework cost associated with modifications was consistently low.