Info

Intermediate node that occurs if a occurs with either b or c following.

a ^ <1 bc^C><e

Intermediate node that occurs when all a, b, and c occur with either d or e following.

Figure 8-25: GERT symbols.

Figure 8-25: GERT symbols.

pass inspection after rework are sent to finishing, a process that requires 10 hours 60 percent of the time and 14 hours otherwise. A final inspection rejects 5 percent of the finished parts, which are then scrapped.

We can now model this situation as a GERT network so that it can be solved for the expected percentage of good parts and the expected time required to produce a good part. This GERT network is illustrated in Figure 8-26.

Activity a represents the output of the four-hour manufacturing process. The outputs enter an inspection from which 75 percent are passed, c, and 25 percent

Finishing Inspect Good parts

Figure 8-26: Sample GERT network.

Finishing Inspect Good parts

Figure 8-26: Sample GERT network.

fail, b. The latter go to rework, with flow d emerging. Another inspection takes place—e (30 percent of 25 percent = 7.5%) flows to scrap, while the successfully reworked parts (70 percent of 25 percent = 17.5 percent), represented byf, go with the other good parts, c, to the finishing process. Sixty percent of this input requires 10 hours of work, g, and the remainder (40 percent) needs 14 hours, h. The final inspection process discards 5 percent of the output, 1 which goes to scrap, and the remainder (which is 87.875 percent of the original input) is sent to "good parts,").

The time for an "average" part to proceed through the network can be found in much the same way as we calculated the output. The result of the entire analysis is therefore considerably richer than the simpler PERT/CPM. It should, however, be obvious that the input information requirements for GERT are more extensive and the computational requirements are far more extensive than for PERT/CPM, particularly for large networks. As always, the PM should adopt the simplest scheduling technique consistent with the needs of the project.

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