The three probabilistic models also are based on the process chart as depicted, but they include all its uncertainties in durations and likelihood of rework. The variability in fabrication duration was taken from Howell and Ballard (1996). A piping industry rework rate ranging from 1 to 10% was quoted by Ballard and the worst value is used here. Lacking other data, rework is assumed to take the same amount of time as fabrication. The three models differ from one another (1) in the draw order of cut sheets for fabrication and (2) only the last model includes the Feedback queue, the Update activity, and links FB1, FB2, DW3, and DW4 (shown in dotted lines in Figure 2).