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SEQUENCING:Assumptions to Priority Rules, Scheduling Service Operations

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Production and Operations Management ­MGT613
VU
Lesson 42
SEQUENCING
Learning Objectives
After completing today's lecture, the students should be able to develop a comprehensive understanding
of scheduling and sequencing operations with the help of Hungarian Methods and Johnson Rules. The
students would be able to understand the priority rules along with the need of effective scheduling and
sequencing. The students would be able to develop an Operations Strategy with respect to both
Scheduling and Sequencing.
Sequencing
Sequencing: Determine the order in which jobs at a work center will be processed.
Requires order for sequencing at all work centers as well as sequencing at individual work centers.
Workstation: An area where one person works, usually with special equipment, on a specialized job.
Sequencing
Job time: Time needed for setup and processing of a job.
Priority rules: Simple heuristics (Commonsense rules) used to select the order in which jobs will be
processed.
1. Local Rules ( pertaining to single workstation)
2. Global Rules( pertaining to multiple workstation)
3. Job processing times and due dates are important pieces of information.
4. Job time consists of processing time and setup times
Priority Rules
1. FCFS - First Come, First Served: Jobs are processed in the order in which they arrive at a
machine or work center.
2. SPT- Shortest Processing Time: Jobs are processed according to processing time at a
machine or work center, shortest job first.
3. DD - Due Date: Jobs are processed according to due date, earliest due date first.
4. CR - critical ratio: Jobs are processed according to smallest ratio of time remaining until
due date to processing time remaining.
5. S/O - slack per operation: Jobs are processed according to average slack time (time until
due date minus remaining time to process). Compute by dividing slack time by dividing
slack time by number of remaining operations including the current one.
6. Rush ­ emergency: Emergency or Preferred Customers first.
Assumptions to Priority Rules
1. The set of jobs is known, no new jobs arrive after processing begins and no jobs are
canceled.
2. Setup time is deterministic
3. Processing times are deterministic rather than variables.
4. There will be no interruptions in processing such as machine breakdowns , accidents or
worker illnesses.
Definitions
Job Flow Time: The length of time a job is in the shop at a particular workstation or work center.
Job Lateness: This is the length of time the job completion date is expected to exceed the date the
job was due or promised to a customer.
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Production and Operations Management ­MGT613
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Makespan: This is the total time needed to complete a group of jobs. It is the length of time between
the start of the first job in the group and the completion of the last job in the group.
Average Number of Jobs: Jobs that are considered in a shop are considered to be work in process
inventory. Mathematically
Average Number of Jobs= Total Flow Time / Makespan
Example
Determine the sequence of jobs, average time flow, average days late and average number of jobs at
the work center, for each of these rules
FCFS
SPT
DD
CR
Example Data
JOB
Processing Time
Due Date
A
2
7
B
8
16
C
4
4
D
10
17
E
5
15
F
12
18
Part A. FCFS
Assume Jobs arrived in the following order 1. A-B-C-D-E-F
Processing
JOB
Flow Time ( cumulative
Due Date (2)-
Sequences
Time
processing time) (2)
(3)
(3)
(1)
A
2
2
7
0
B
8
10
16
0
C
4
14
4
10
D
10
24
17
7
E
5
29
15
14
F
12
41
18
22
41
120
54
Part A. FCFS
Average Flow time= Total Flow Time/Number of Jobs=120/6=20 days
Average Tardiness=54/6=9
The makespan =41 days
Average Number of Jobs at workstation=
120/41=2.93 jobs per workstation
SPT, the sequence is A-C-E-B-D-F
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Production and Operations Management ­MGT613
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Part B SPT rule
JOB
Processing
Flow Time ( cumulative
Due
(2)-(3)
Sequences
Time
processing time)
Date
(1)
(2)
(3)
A
2
2
7
0
C
4
6
4
2
E
5
11
15
0
B
8
19
16
3
D
10
29
17
12
F
12
41
18
23
41
108
40
Average Flow time= Total Flow Time/Number of Jobs=108/6=18 days
Average Tardiness=40/6=6.67days
The makespan =41 days
Average Number of Jobs at workstation=
108/41=2.63 jobs per workstation
Summary Part A,B,C and D
Average Average  Average Number of Jobs of
JOB
Flow
Lateness  the Work Center
Sequences
Time
(Days)
Rule
(Days)
FCFS
20.00
9.00
2.93
SPT
18.00
6.67
2.63
DD
18.33
6.33
2.68
CR
26.67
14.17
3.9
Summary Part A,B,C and D
1. Generally Speaking FCFS and CR rule seems to be the least effective.
2. CR is the worst in each aspect of measurement.
3. The primary limitation of FCFS is that long jobs will tend to delay other jobs.
4. However in scheduling of service systems, the FCFS has the advantage of simplicity,
inherent fairness (first come first served) but also due to non availability of realistic
estimates of processing times for individual jobs.
Johnson's Rule (Two Work Center Sequencing)
Johnson's Rule: technique for minimizing completion time for a group of jobs to be processed on
two machines or at two work centers.
1. Minimizes total idle time
2. Several conditions must be satisfied
Johnson's Rule Conditions
1.
Job time must be known and constant
2.
Job times must be independent of sequence
3.
Jobs must follow same two-step sequence
4.
Job priorities cannot be used
5.
All units must be completed at the first work center before moving to second
Johnson's Rule Optimum Sequence
1. List the jobs and their times at each work center
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2. Select the job with the shortest time
3. Eliminate the job from further consideration
4. Repeat steps 2 and 3 until all jobs have been scheduled
Johnson's Rule Example
PROCESSING TIMES ( HOURS)
JOB
Work Center 1
Work Center2
A
5
5
B
4
3
C
8
9
D
2
7
E
6
8
F
12
15
1. Select the job with shortest processing time. In our case it is Job D
2. Eliminate the row of JOB D & proceed further to select the next job with shortest
processing time which is B in our case and that too at work center2
3. Similarly sequencing other jobs at work centers, we follow that if there is tie of no of hours,
we can sequence it at the beginning or the end.
4. Construct a chart to determine the throughput time and idle times at the work centers. We
have
5. 1st D,2nd E,3rd C,4th F,5th A and 6th B
Scheduling Difficulties
1. Variability in
a. Setup times
b. Processing times
c. Interruptions
d. Changes in the set of jobs
2. No method for identifying optimal schedule
3. Scheduling is not an exact science
4. Ongoing task for a manager
Minimizing Scheduling Difficulties
1. Set realistic due dates
2. Focus on bottleneck operations
3. Consider lot splitting of large jobs
Scheduling Service Operations
1. Appointment systems
a. Controls customer arrivals for service
2. Reservation systems
a. Estimates demand for service
3. Scheduling the workforce
a. Manages capacity for service
4. Scheduling multiple resources
a. Coordinates use of more than one resource
Cyclical Scheduling
1.
Hospitals, police/fire departments, restaurants, supermarkets
2.
Rotating schedules
3.
Set a scheduling horizon
4.
Identify the work pattern
5.
Develop a basic employee schedule
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6. Assign employees to the schedule
Service Operation Problems
1. Cannot store or inventory services
2. Customer service requests are random
3. Scheduling service involves
a. Customers
b. Workforce
c. Equipment
Maintenance
Maintenance: All activities that maintain facilities and equipment in good working order so that a
system can perform as intended
Breakdown maintenance: Reactive approach; dealing with breakdowns or problems when they
occur
Preventive maintenance: Proactive approach; reducing breakdowns through a program of
lubrication, adjustment, cleaning, inspection, and replacement of worn parts
Maintenance Reasons
Reasons for keeping equipment running
1. Avoid production disruptions
2. Not add to production costs
3. Maintain high quality
4. Avoid missed delivery dates
Breakdown Consequences
Some of the Important consequences are:
1. Production capacity is reduced : Orders are delayed
2. No production: Overhead continues
3. Cost per unit increases: Quality issues
4. Product may be damaged
5. Safety issues
a. Injury to employees
b. Injury to customers
Total Maintenance Cost
Total
Cost
Preventive
maintenance cost
Breakdown and
repair cost
Optimum Amount of
Preventive Maintenance
Preventive Maintenance
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Preventive maintenance: goal is to reduce the incidence of breakdowns or failures in the plant or
equipment to avoid the associated costs
1. Preventive maintenance is periodic
2. Result of planned inspections
3. According to calendar
4. After predetermined number of hours
Example 1
Frequency of breakdown
1
2
3
Number of breakdowns
0
Frequency of occurrence
.20 .30 .40 .10
If the average cost of a breakdown is Rs.10,000, and the cost of preventative maintenance is
Rs.12,500 per month, should we use preventive maintenance?
Example 1 Solution
Number of
Frequency
Expected
Breakdowns
of
number of
Occurrence
Breakdowns
0
.20
0
1
.30
.30
2
.40
.80
3
.10
.30
1.00
1.40
Expected cost to repair = 1.4 breakdowns per month X Rs.10,000
= Rs.14,000
Preventive maintenance = Rs.12,500.
PM results in savings of Rs.1500 per month
Predictive Maintenance
Predictive maintenance: An attempt to determine when best to perform preventive maintenance
activities
Total productive maintenance: JIT approach where workers perform preventive maintenance on the
machines they operate
Breakdown Programs
1.
Standby or backup equipment that can be quickly pressed into service
2.
Inventories of spare parts that can be installed as needed
3.
Operators who are able to perform minor repairs
4.
Repair people who are well trained and readily available to diagnose and correct problems
with equipment
Replacement
1.
Trade-off decisions
2.
Cost of replacement vs. cost of continued maintenance
3.
New equipment with new features vs. maintenance
4.
Installation of new equipment may cause disruptions
5.
Training costs of employees on new equipment
6.
Forecasts for demand on equipment may require new equipment capacity
7.
When is it time for replacement?
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Operations Strategy
1. Scheduling can hinder or help the Operations Strategy.
2. An on time delivery of a product or service is only possible if the Operations Managers is
able to do effective scheduling.
3. An ineffective scheduling would result in inefficient use of resources and possible
dissatisfied customers.
4. Scheduling as an Operations Strategy can provide an organization a competitive advantage
over its competitors.
5. Time based competition depends on good scheduling.
6. Good design, superior quality and other elements of a well run organization are meaningless
if effective scheduling is absent from Operations Management Strategy.
7. Scheduling is that bank balance which may seem great in numbers but if not used
effectively would not make any sense.
Summary
Scheduling involves timing and coordination of operations. Scheduling is different for high volume,
intermediate volume and low volume. Scheduling for job shops is very complex, because of variety
jobs that need to be processed. The two major problems in Scheduling is the assigning of jobs to the
machines (work centers) and sequence of operations at a machine.
Gantt Load charts are used to help managers visualize the work load situation.
Scheduling of service systems require appointment or reservation systems, although all systems are
not amenable to this. When multiple resources are involved, the balancing can be difficult.
Maintenance Activities ensure that sequence activities are completed on time and as per schedule.
Maintenance adds to revenue and not to costs.
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Table of Contents:
  1. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT
  2. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT:Decision Making
  3. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT:Strategy
  4. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT:Service Delivery System
  5. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT:Productivity
  6. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT:The Decision Process
  7. INTRODUCTION TO PRODUCTION AND OPERATIONS MANAGEMENT:Demand Management
  8. Roadmap to the Lecture:Fundamental Types of Forecasts, Finer Classification of Forecasts
  9. Time Series Forecasts:Techniques for Averaging, Simple Moving Average Solution
  10. The formula for the moving average is:Exponential Smoothing Model, Common Nonlinear Trends
  11. The formula for the moving average is:Major factors in design strategy
  12. The formula for the moving average is:Standardization, Mass Customization
  13. The formula for the moving average is:DESIGN STRATEGIES
  14. The formula for the moving average is:Measuring Reliability, AVAILABILITY
  15. The formula for the moving average is:Learning Objectives, Capacity Planning
  16. The formula for the moving average is:Efficiency and Utilization, Evaluating Alternatives
  17. The formula for the moving average is:Evaluating Alternatives, Financial Analysis
  18. PROCESS SELECTION:Types of Operation, Intermittent Processing
  19. PROCESS SELECTION:Basic Layout Types, Advantages of Product Layout
  20. PROCESS SELECTION:Cellular Layouts, Facilities Layouts, Importance of Layout Decisions
  21. DESIGN OF WORK SYSTEMS:Job Design, Specialization, Methods Analysis
  22. LOCATION PLANNING AND ANALYSIS:MANAGING GLOBAL OPERATIONS, Regional Factors
  23. MANAGEMENT OF QUALITY:Dimensions of Quality, Examples of Service Quality
  24. SERVICE QUALITY:Moments of Truth, Perceived Service Quality, Service Gap Analysis
  25. TOTAL QUALITY MANAGEMENT:Determinants of Quality, Responsibility for Quality
  26. TQM QUALITY:Six Sigma Team, PROCESS IMPROVEMENT
  27. QUALITY CONTROL & QUALITY ASSURANCE:INSPECTION, Control Chart
  28. ACCEPTANCE SAMPLING:CHOOSING A PLAN, CONSUMER’S AND PRODUCER’S RISK
  29. AGGREGATE PLANNING:Demand and Capacity Options
  30. AGGREGATE PLANNING:Aggregate Planning Relationships, Master Scheduling
  31. INVENTORY MANAGEMENT:Objective of Inventory Control, Inventory Counting Systems
  32. INVENTORY MANAGEMENT:ABC Classification System, Cycle Counting
  33. INVENTORY MANAGEMENT:Economic Production Quantity Assumptions
  34. INVENTORY MANAGEMENT:Independent and Dependent Demand
  35. INVENTORY MANAGEMENT:Capacity Planning, Manufacturing Resource Planning
  36. JUST IN TIME PRODUCTION SYSTEMS:Organizational and Operational Strategies
  37. JUST IN TIME PRODUCTION SYSTEMS:Operational Benefits, Kanban Formula
  38. JUST IN TIME PRODUCTION SYSTEMS:Secondary Goals, Tiered Supplier Network
  39. SUPPLY CHAIN MANAGEMENT:Logistics, Distribution Requirements Planning
  40. SUPPLY CHAIN MANAGEMENT:Supply Chain Benefits and Drawbacks
  41. SCHEDULING:High-Volume Systems, Load Chart, Hungarian Method
  42. SEQUENCING:Assumptions to Priority Rules, Scheduling Service Operations
  43. PROJECT MANAGEMENT:Project Life Cycle, Work Breakdown Structure
  44. PROJECT MANAGEMENT:Computing Algorithm, Project Crashing, Risk Management
  45. Waiting Lines:Queuing Analysis, System Characteristics, Priority Model