Supplement
J Operations Scheduling
DISCUSSION QUESTION
1. The optimizing approach, of course, would give the optimal schedule for a group of
jobs. However, implementing the model would be difficult. For example, significant
amounts of data would need to be maintained and updated each time the model was
used. There would also likely be circumstances when the schedule would have to be
manually adjusted to account for unexpected happenings. Of course, the model’s
assumptions (linearity or nonlinearity, deterministic or stochastic, and so forth) could
come into serious question.
The priority sequencing rule approach does not claim to provide an optimal
solution, but it is much easier to implement and “adjusts” to unexpected happenings
as they occur. The optimizing approach might prove to be the better choice in
environments where there are few new job arrivals during the week (or they can be
held until the next scheduling session) and there are few unexpected disruptions to the
process. The sequencing rule approach is likely to be the better choice in dynamic
environments where control of the schedule is difficult without making changes
periodically.
Technology and software advances for real-time scheduling may offer the best of
both approaches.
PROBLEMS
Job Shop Scheduling
1. Hickory Company
a. FCFS: SPT: EDD:
Job Hr Since Finish Flow Job Hr Since Finish Flow Job Hr Since Finish Flow
Order Time Time Order Time Time Order Time Time
Arrived Arrived Arrived
1 12 10 22 2 10 3 13 2 10 3 13
2 10 13 23 5 1 10 11 1 12 13 25
3 7 28 35 4 3 19 22 3 7 28 35
4 3 37 40 1 12 29 41 4 3 37 40
5 1 44 45 3 7 44 51 5 1 44 45
Total 165 138 158
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J-1
Copyright © 2016 Pearson Education, Inc.
, J-2 l Supplement J l Operations Scheduling
b. FCFS: SPT: EDD:
Average flow times 33.0 27.6 31.6
Average early time 0.4 3.4 1.0
Average past due 11.0 8.6 10.2
c. The rules perform as expected, except for SPT on the average past due measure.
Typically EDD will do better here. Nonetheless, SPT does well on flow times.
2. Drill press
a., b. The following tables give the solutions to parts (a) and (b) using the Single-
Workstation Scheduler from OM Explorer.
Note: OM Explorer prints out not just the Average Flow Times, but also the
Average Early Time and Average Past Due. These last two performance measures
are not shown here, because they are inappropriate. The reason is that times to due
date were expressed in terms of weeks, whereas the times since the jobs arrived
and the processing times of the jobs are expressed in terms of hours. The same
unit of time is needed for correct statistics on the last two performance measures.
c. Priority planning with an MRP system relies on proper timing of materials.
Planners manipulate scheduled due dates to match material need dates with order
due dates. Consequently, priority rules incorporating due dates would be most
useful in communicating these changes to the shop floor. Of those listed in this
problem, EDD, S/RO, and CR would work best.
Copyright © 2016 Pearson Education, Inc.
J Operations Scheduling
DISCUSSION QUESTION
1. The optimizing approach, of course, would give the optimal schedule for a group of
jobs. However, implementing the model would be difficult. For example, significant
amounts of data would need to be maintained and updated each time the model was
used. There would also likely be circumstances when the schedule would have to be
manually adjusted to account for unexpected happenings. Of course, the model’s
assumptions (linearity or nonlinearity, deterministic or stochastic, and so forth) could
come into serious question.
The priority sequencing rule approach does not claim to provide an optimal
solution, but it is much easier to implement and “adjusts” to unexpected happenings
as they occur. The optimizing approach might prove to be the better choice in
environments where there are few new job arrivals during the week (or they can be
held until the next scheduling session) and there are few unexpected disruptions to the
process. The sequencing rule approach is likely to be the better choice in dynamic
environments where control of the schedule is difficult without making changes
periodically.
Technology and software advances for real-time scheduling may offer the best of
both approaches.
PROBLEMS
Job Shop Scheduling
1. Hickory Company
a. FCFS: SPT: EDD:
Job Hr Since Finish Flow Job Hr Since Finish Flow Job Hr Since Finish Flow
Order Time Time Order Time Time Order Time Time
Arrived Arrived Arrived
1 12 10 22 2 10 3 13 2 10 3 13
2 10 13 23 5 1 10 11 1 12 13 25
3 7 28 35 4 3 19 22 3 7 28 35
4 3 37 40 1 12 29 41 4 3 37 40
5 1 44 45 3 7 44 51 5 1 44 45
Total 165 138 158
-
J-1
Copyright © 2016 Pearson Education, Inc.
, J-2 l Supplement J l Operations Scheduling
b. FCFS: SPT: EDD:
Average flow times 33.0 27.6 31.6
Average early time 0.4 3.4 1.0
Average past due 11.0 8.6 10.2
c. The rules perform as expected, except for SPT on the average past due measure.
Typically EDD will do better here. Nonetheless, SPT does well on flow times.
2. Drill press
a., b. The following tables give the solutions to parts (a) and (b) using the Single-
Workstation Scheduler from OM Explorer.
Note: OM Explorer prints out not just the Average Flow Times, but also the
Average Early Time and Average Past Due. These last two performance measures
are not shown here, because they are inappropriate. The reason is that times to due
date were expressed in terms of weeks, whereas the times since the jobs arrived
and the processing times of the jobs are expressed in terms of hours. The same
unit of time is needed for correct statistics on the last two performance measures.
c. Priority planning with an MRP system relies on proper timing of materials.
Planners manipulate scheduled due dates to match material need dates with order
due dates. Consequently, priority rules incorporating due dates would be most
useful in communicating these changes to the shop floor. Of those listed in this
problem, EDD, S/RO, and CR would work best.
Copyright © 2016 Pearson Education, Inc.
