1. Operations Management Short-Term Scheduling 短期批程 Chapter 15
Week #16-2

2. Outline GLOBAL COMPANY PROFILE: DELTA AIRLINES
THE STRATEGIC IMPORTANCE OF SHORT-TERM SCHEDULING (短期批程在策略上的重要性)
SCHEDULING ISSUES (批程問題)
Forward and Backward Scheduling
Scheduling Criteria
SCHEDULING PROCESS-FOCUSED WORK CENTERS

3. Outline - Continued LOADING JOBS IN WORK CENTERS
Input-Output Control
Gantt Charts
Assignment Method
SEQUENCING JOBS IN WORK CENTERS
Priority Rules for Dispatching Jobs
Critical Ratio
Sequencing N Jobs on Two Machines: Johnson’s Rule

4. Outline - Continued LIMITATIONS OF RULE-BASED DISPATCHING SYSTEMS
FINITE SCHEDULING
THEORY OF CONSTRAINTS
BOTTLENECK WORK CENTERS
REPETITIVE MANUFACTURING
SCHEDULING FOR SERVICES
Scheduling Service Employees with Cyclical Scheduling

5. Learning Objectives
When you complete this chapter, you should be able to :
Identify or Define:
Gantt charts
Assignment method
Sequencing rules
Johnson’s rules
Bottlenecks

6. Learning Objectives - continued
When you complete this chapter, you should be able to :
Describe or Explain:
Scheduling
Sequencing
Shop loading
Theory of constraints

7. Delta Airlines
10% of Delta’s flights disrupted per year - half of those by weather
Cost: $440-million in:
lost revenue
overtime pay
food and lodging vouchers
$33-million hi-tech nerve center
18 staff
$35-million savings
One approach to take in discussing Delta Airlines is to ask students to describe Delta’s short term problem - and to be sure to include as many factors as possible. Very likely, the first attempts will be fairly short. Students will probably need prodding - not to identify the factors, but to include them. Once the list has been made, ask the students to explain how they would like to account for these factors in their scheduling model.
Another question for students: Is the problem for Delta, as discussed in the case, one of developing a schedule under a complex set of constraints, or, rather, one of devising a minimal set of modifications to an already developed schedule?

8. Strategic Implications of Short-Term Scheduling
By scheduling effectively, companies use assets more effectively and create greater capacity per dollar invested, which, in turn, lowers cost
This added capacity and related flexibility provides faster delivery and therefore better customer service
Good scheduling is a competitive advantage which contributes to dependable delivery
You might stress to students that scheduling is what the end-customer actually sees.

9. Short-Term Scheduling
Deals with timing of operations
Short run focus: Hourly, daily, weekly
Types
Today
Due Date B E
Forward Scheduling
Backward Scheduling

10. Short-Term Scheduling Examples
© T/Maker Co.
Hospital
Outpatient treatments
Operating rooms
University
Instructors
Classrooms
Factory
Production
Purchases
Students should be asked to provide additional examples. They should also be asked to identify any “unusual circumstances” which might pertain to their examples.

11. Scheduling Decisions Organization Managers Must Schedule
Operating room use
Patient admission
Nursing, security, maintenance staffs
Outpatient treatments
Classrooms and audiovisual equipment
Student and instructor schedules
Graduate and undergraduate courses
Production of goods
Purchase of materials
Workers
Mount Sinai Hospital
Indiana University
Lockheed-Martin Factory
Here you might ask students to identify the scheduling decisions for a business with which they are familiar. McDonalds, grocery store, and doctor’s office are useful examples.

12. Scheduling Decisions Organization Managers Must Schedule
Chefs, waiters,bartenders
Delivery of fresh foods
Entertainers
Opening of dining areas
Maintenance of aircraft
Departure timetables
Flight crews, catering, gate, and ticketing personnel
Hard Rock Cafe
Delta Airlines

13. Capacity Planning, Aggregate Scheduling, Master Schedule, and Short-Term Scheduling
1. Facility size
2. Equipment procurement
Long-term
Aggregate Scheduling
1. Facility utilization
2. Personnel needs
3. Subcontracting
Intermediate-term
Master Schedule
1. MRP
2. Disaggregation of master plan
Intermediate-term
Students should be asked to identify three decisions at each level.
Short-term Scheduling
1. Work center loading
2. Job sequencing
Short-term

14. Forward and Backward Scheduling
Forward scheduling: begins the schedule as soon as the requirements are known
jobs performed to customer order
schedule can be accomplished even if due date is missed
often causes buildup of WIP
Backward scheduling: begins with the due date of the final operation; schedules jobs in reverse order
used in many manufacturing environments, catering, scheduling surgery
Students should be asked to consider how, using either process, they would modify the schedule when (a) an order is cancelled, (b) a priority, rush order is accepted, or (c) a delay is encountered.

15. The Goals of Short-Term Scheduling
Minimize completion time
Maximize utilization (make effective use of personnel and equipment)
Minimize WIP inventory (keep inventory levels low)
Minimize customer wait time
While we would like to minimize customer wait time, an alternate goal might be to make customer wait time consistent - we could always ask them to come later.

16. Choosing a Scheduling Method
Qualitative factors
Number and variety of jobs
Complexity of jobs
Nature of operations
Quantitative criteria
Average completion time
Utilization (% of time facility is used)
WIP inventory (average # jobs in system)
Customer waiting time (average lateness)
Ask students to identify at least one example of a scheduling based on qualitative factors, and one one quantitative factors.

17. Scheduling Methods Differ by Process
Level Use Methods
Repetitive-Focused
Product-Focused
Process-Focused
Variety of Methods

18. Process-Focused Work Centers
High variety, low volume systems
Products made to order
Products need different materials and processing
Complex production planning and control
Production planning aspects
Shop loading
Job sequencing

19. Requirements for Scheduling Process-Focused Work Centers
Schedule incoming orders without violating capacity constraints of individual work centers
Check availability of tools and materials before releasing an order to a department
Establish due dates for each job and check progress against need dates and order lead times
Check work-in-progress as jobs move through the shop
Provide feedback on plant and production activities
Provide work-efficiency statistics and monitor operator times for payroll and labor distribution analyses

20. Types of Planning Files
Item master file - contains information about each component the firm produces or purchases
Routing file - indicates each component’s flow through the shop
Work-center master file - contains information about the work center such as capacity and efficiency
It is helpful to discuss the purpose of each of the planning files. What is the data used for?

21. Process-Focused Planning System
Aggregate
Forecast &
Resource
Production
Firm Orders
Availability
Planning
Material
Master
Requirements
Production
Planning
Scheduling
No, modify CRP, MRP, or MPS
Capacity
Shop
It is helpful to actually walk the students through the steps illustrated in this slide.
Requirements
Realistic?
Floor
Yes
Planning
Schedules

22. Loading Jobs in Work Centers
Assigning jobs to work centers
Considerations
Job priority (e.g., due date)
Capacity
Work center hours available
Hours needed for job
Approaches
Gantt charts (load & scheduling) - capacity
Assignment method - job to specific work center

23. Input-Output Control Week Ending 6/6 6/13 6/20 6/27 7/4 7/11
Planned input
280
Actual input
270
250
285
Cumulative Deviation
-10
-40
-35
Planned Output
320
Actual Output
-50
-100
-150
-200
Cumulative Change in Backlog
-20
-+5
Explanation: 270 input, 270 output, implies 0 change
Explanation: 250 input, 270 output implies –20 change (20 standard hours less in the work center)

24. Order Release Begins Shop LoadingNo
Release Order
Hold Release
Priority & Capacity OK?
Gross Requirements
Scheduled Receipts
Available
Net Requirements
Planned Order Receipt
Planned Order Release
Yes 1 2
100 3

25. Options for Managing Facility Work Flow
Correcting performance
Increasing capacity
Increasing or reducing input to the work center by:
routing work to or from other work centers
increasing or decreasing subcontracting
producing less (or more)
Students should be asked to consider which of these options allow the most flexibility.

26. Gantt Load Chart Shows relative workload in facility Disadvantages
Does not account for unexpected events
Must be updated regularly
Work Center M T W Th F
Metal Works
Job 349
Job 350
Mechanical
Job D
Job G
Electronics
Job B
Job H
Painting
Job C
Job E
Job I
A characteristic of most of our project management and schedule management tools is that they require management attention for updating on a regular basis.

27. Gantt Load Chart for Week of March 6
Shows relative workload in facility
Disadvantages
Does not account for unexpected events
Must be updated regularly
Day
Work Center
Monday
Tuesday
Wednesday
Thursday
Friday
Metalworks
Job 349
Job 350
Mechanical
Job 406
Electronics
Job 408
Painting
Job 295
Center not available (for example, maintenance time, repairs, shortages)
Processing
Unscheduled

28. Gantt Scheduling Chart
Start of an activity
Scheduled activity time allowed
Point in time when chart is reviewed S T W F
Day
Job
Job A
Job B
Job C
Maintenance
Now
End of an activity
Actual work progress
Non-production time
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7

29. Assignment Method Assigns tasks or jobs to resources
Type of linear programming model
Objective
Minimize total cost, time etc.
Constraints
1 job per resource (e.g., machine)
1 resource (e.g., machine) per job
Students should be asked to consider what other assumptions must be made when using the assignment model for job scheduling.

30. Assignment Method - Four Steps
Subtract the smallest number in each row from every number in that row; then subtract the smallest number in every column from every number in that column
Draw the minimum number of vertical and horizontal straight lines necessary to cover all zeros in the table
If the number of lines equals either the number of rows or the number of columns, then you can make an optimal assignment (Step 4)
Otherwise:
Subtract the smallest number not covered by a line from every other uncovered number. Add the same number to any number(s) lying at the intersection of any two lines. Return to Step 2
Optimal assignments will always be at the zero locations of the table
It should be noted to students that there are other methodologies available for solving the assignment problem (branch-and-bound, etc.)

31. Assignment Method – Type Setter Example
Job A B C
R-34
$11
$14
$ 6
S-66
$ 8
$10
T-50 $9
$12 $7
Initial set-up

32. Step 1a & 1b Step 1a Step 1b Typesetter Job A B C R-34 5 8 S-66 2 3
S-66 2 3
T-50
Typesetter
Job A B C
R-34 5 6
S-66 3
T-50 2
Step 1a
Step 1b

33. Step 2 Smallest uncovered number Typesetter Job A B C R-34 5 6 S-66 3
S-66 3
T-50 2
Smallest uncovered number

34. Step 3 Make assignments R-34 →C, S-66S → B, T-50 →A Typesetter Job A B
S-66 5
T-50 1
Make assignments
R-34 →C, S-66S → B, T-50 →A

35. Sequencing Challenge Order release Job Packet Job XYZ
Which job do I run next?
Dispatch List
Order Part Due Qty
XYZ
ABC
Production Control
Production

36. Sequencing Specifies order jobs will be worked Sequencing rules
First come, first served (FCFS)
Shortest processing time (SPT)
Earliest due date (EDD)
Longest processing time (LPT)
Critical ratio (CR)
Johnson’s rule
The next seven slides provide additional discussion of these rules.

37. Priority Rules for Dispatching Jobs
First come, first served
The first job to arrive at a work center is processed first
Earliest due date
The job with the earliest due date is processed first
Shortest processing time
The job with the shortest processing time is processed first
Longest processing time
The job with the longest processing time is processed first
Critical ratio
The ratio of time remaining to required work time remaining is calculated, and jobs are scheduled in order of increasing ratio.
FCFS
EDD
SPT
LPT CR

38. First Come, First Served Rule
Process first job to arrive at a work center first
Average performance on most scheduling criteria
Appears ‘fair’ & reasonable to customers
Important for service organizations
Example: Restaurants

39. Shortest Processing Time Rule
Process job with shortest processing time first.
Usually best at minimizing job flow and minimizing the number of jobs in the system
Major disadvantage is that long jobs may be continuously pushed back in the queue.

40. Longest Processing Time Rule
Process job with longest processing time first.
Usually the least effective method of sequencing.

41. Earliest Due Date Rule Process job with earliest due date first
Widely used by many companies
If due dates important
If MRP used
Due dates updated by each MRP run
Performs poorly on many scheduling criteria

42. Critical Ratio (CR) Ratio of time remaining to work time remaining=
Work days
remaining
Due date -
Today'
s date =
Work (lead
) time remaining
Process job with smallest CR first
Performs well on average lateness

43. Advantages of the Critical Ratio Scheduling Rule
Use of the critical ratio can help to:
determine the status of a specific job
establish a relative priority among jobs on a common basis
relate both stock and make-to-order jobs on a common basis
adjust priorities and revise schedules automatically for changes in both demand and job progress
dynamically track job progress and location

44. Criteria to Evaluate Priority Rules
You might point out (even though you have probably noted it before) that ratings or rankings of the different scheduling rules will differ depending upon the criteria used.

45. Job Sequencing Example
Job Work
Processing time in days
Job Due Date (day) A 6 8 B 2 C 18 D 3 15 E 9 23

46. FCFS Parameter Value Average completion time 15.4 days Utilization
36.4%
Average number of jobs in the system
2.75 jobs
Average job lateness
2.2 days
Sequence A B C D E

47. SPT Parameter Value Average completion time 13 days Utilization 43.1%
Average number of jobs in the system
2.32 jobs
Average job lateness
1.8 days
Sequence B D A C E

48. EDD Parameter Value Average completion time 13.6 days Utilization
41.2%
Average number of jobs in the system
2.43 jobs
Average job lateness
1.2 days
Sequence B A D C E

49. LPT Parameter Value Average completion time 20.6 days Utilization
27.2%
Average number of jobs in the system
3.68 jobs
Average job lateness
9.6 days
Sequence E C A D B

50. Summary Rule Average Completion Time (days) Utilization (%)
Average Number of Jobs in the System
Average Lateness (Days)
FCFS
15.4
36.4
2.75
2.2
SPT
13.0
43.1
2.32
1.8
EDD
13.6
41.2
2.43
1.2
LPT
20.6
27.2
3.68
9.6

51. Processing time in days
Critical Ratio (CR)
Job
Job Work
Processing time in days
Job Due Date (day)
Critical Ratio A 6 8
0.75 B 2
0.33 C 18
0.44 D 3 15
0.20 E 9 23
0.39
Sequence A C E B D

52. Johnson’s Rule
Used to sequence N jobs through 2 machines in the same order
© 1995 Corel Corp.
Saw
Drill
Job A
Job B
Job C
Jobs (N = 3)

53. Johnson's Rule - Scheduling N Jobs on Two Machines
All jobs are to be listed, and the time each requires on a machine shown.
Select the job with the shortest activity time. If the shortest time lies with the first machine, the job is scheduled first; if with the second machine, the job is scheduled last.
Once a job is scheduled, eliminate it.
Apply steps 2-3 to the remaining jobs, working toward the center of the sequence.

54. Johnson’s Rule Steps 2 1 List jobs & activity times Select job with
shortest time
Machine?
Schedule
FIRST
LAST
Eliminate job
from list
Jobs left?
Break
arbitrarily
Ties?
Yes 1 2
Stop No

55. Johnson’s Rule - Example
Job
Work Center 1
(Drill Press)
Work Center 2
(Lathe) A 5 2 B 3 6 C 8 4 D 10 7 E 12

56. Johnson’s Rule - Example
Step 1 A
Step 2 B A
Step 3 B C A
Step 4 B D C A
Step 5 B E D C A

57. Graphical Depiction of Job Flow
Work center 1
Work center 2
Time => B E D C A B E D C A
= Idle
= Job completed

58. Limitations of Rule-Based Dispatching Systems
Scheduling is dynamic; therefore, rules need to be revised to adjust to changes in process, equipment, product mix, etc.
Rules do not look upstream or downstream; idle resources and bottleneck resources in other departments may not be recognized
Rules do not look beyond due dates
You might point out to students that this is another case of models which are insufficiently robust to handle all factors.
Ask students to consider how these limitations might be overcome. (Finite scheduling?) What is the role of information technology?

59. Finite Scheduling System

60. Theory of Constraints
Deals with factors limiting company’s ability to achieve goals
Types of constraints
Physical
Example: Machines, raw materials
Non-physical
Example: Morale, training
Limits throughput in operations

61. Theory of Constraints A Five Step Process
Identify the constraints
Develop a plan for overcoming the identified constraints
Focus resources on accomplishing the constraints identified in step 2
Reduce the effects of the constraints by off-loading work or by expanding capability
Once one set of constraints is overcome, return to the first step and identify new constraints

62. Bottleneck Work Centers
Bottleneck work centers have less capacity than prior or following work centers
They limit production output
© 1995 Corel Corp.

63. Techniques for Dealing With Bottlenecks
Increase the capacity of the constraint
Ensure well-trained and cross-trained employees are available to operate and maintain the work center causing the constraint
Develop alternate routings, processing procedures, or subcontractors
Move inspections and tests to a position just before the constraint
Schedule throughput to match the capacity of the bottleneck

64. The 10 Commandments for Correct Scheduling
Utilization of a non-bottleneck resource is determined not by its own capacity but by some other constraint in the system
Activating a resource is not synonymous with utilizing a resource
An hour lost at a bottleneck is an hour lost of the whole system
An hour saved at a non-bottleneck is a mirage
The transfer batch may not, and many times should not, be equal to the process batch
Discussion should include not only a definition of the commandment but an explanation of how it impacts the production system.

65. The 10 Commandments for Correct Scheduling
The amount processed should be verifiable and not fixed
Capacity and priority need to be considered simultaneously, not sequentially
Damage from unforeseen problems can be isolated and minimized
Plant capacity should not be balanced
The sum of the local optimums is not equal to the global optimum

66. Repetitive Manufacturing - Advantages of Level Material Use
Lower inventory levels, releasing capital for other uses
Faster product throughput
Improved component quality and hence improved product quality
Reduced floor space requirements
Improved communication among employees because they are closer together
Smoother production process because large lots have not “hidden” the problems

67. Scheduling for Services
Appointment systems - doctor’s office
Reservations systems - restaurant, car rental
First come, first served - deli
Most critical first - hospital trauma room
Students should be asked to consider what special problems arise in scheduling services.

68. Cyclical Scheduling
Plan a schedule equal in weeks to the number of people being scheduled
Determine how many of each of the least desirable off-shifts must be covered each week
Begin the schedule for one worker by scheduling the days off during the planning cycle (at a rate of 2 days per week on average)

69. Cyclical Scheduling - Continued
Assign off-shifts for the first worker
Repeat this pattern for each other worker, but offset by one week from the previous
Allow each worker to pick his/her “slot” or “line” in order of seniority
Mandate that any changes from the chosen schedule are strictly between the personnel wanting to switch