Transcript (Textbook) Behavior in Organizations, 8ed (A. B. Shani)

Capacity Planning, Aggregate Scheduling,
Master Schedule, and Short-Term Scheduling
Capacity Planning
1. Facility size
2. Equipment procurement
Aggregate Scheduling
1. Facility utilization
2. Personnel needs
3. Subcontracting
Master Schedule
1. MRP
2. Dis-aggregation of master plan
Short-term Scheduling
1. Work center loading
2. Job sequencing
Long-term
Intermediate-term
Intermediate-term
Short-term
Scheduling Low-Volume Systems
• Loading - assignment of jobs to
process centers
• Sequencing - determining the
order in which jobs will be
processed
• Job-shop scheduling
– Scheduling for low-volume
systems with many variations
in requirements
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
Loading
• Two Approaches to loading:
- Finite Loading – limit the amount of work
that is released to a given work center on
the basis of capacity consideration.
Input/output control is a method for
keeping track of assigned resource (input)
and the work completed by a resources
(output). This helps identify a backlog
(input rate in greater that output rate). The
remove a backlog to slow down the input.
Loading
- Infinite loading – assign job to center
regardless of available capacity. This
requires that proper planning has been
done to make sure enough capacity is
available to take care of all work to be
done.
Scheduling
• Definition: Determination of when something
is to be done and the tasks and activities
required to do it. Establishing the timing of the
use of equipment, facilities and human
activities in an organization
–Scheduling aids in on-time completion
• Direct link to value perceived by customers
–Scheduling improves the utilization of the firm’s
resources
• Direct link to productivity
8-6
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
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
Scheduling Services
Considerations
• Scheduling services different from
manufacturing
– Inability to store or inventory services
– Random nature of customer requests for
service
• Point of customer contact
– Back-office operations
– Front-office operations
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)
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
Scheduling Difficulties
• Variability in
– Setup times
– Processing times
– Interruptions
– Changes in the set of jobs
• No method for identifying optimal
schedule
• Scheduling is not an exact science
• Ongoing task for a manager
Minimizing Scheduling Difficulties
• Set realistic due dates
• Focus on bottleneck operations
• Consider lot splitting of large jobs
Sequencing
• Sequencing: Determine the order in
which jobs at a work center will be
processed.
• Priority rules: Simple heuristics
used to select the order in
which jobs will be processed.
• Job time: Time needed for
setup and processing of a job.
Priority Rules for Dispatching Jobs
FCFS
First come, first served
The first job to arrive at a work center is processed first
EDD Earliest due date
The job with the earliest due date is processed first
SPT Shortest processing time
The job with the shortest processing time is processed
first
LPT Longest processing time
C
R
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.
Assumptions of Priority Rules
• The setup of jobs is known
• Setup time is independent of processing
sequence
• Setup time is deterministic
• There will be no interruptions in
processing such as:
– Machine breakdowns
– Accidents
– Worker illness
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
SPT and LPT Rule
• SPT – Schedule a job with shortest
processing time first
• LPT – Schedule a job with longest
processing
time first
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
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
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.
• Minimizes total idle time
• Several conditions must be satisfied
Johnson’s Rule Conditions
• Job time must be known and constant
• Job times must be independent of
sequence
• Jobs must follow same two-step
sequence
• Job priorities cannot be used
• All units must be completed at the first
work center before moving to second
Johnson's Rule - Scheduling N
Jobs on Two Machines
1. List all jobs and their processing times each machine.
 2. Select first the job with the shortest processing time. If
the shortest processing time lies with the first machine, the
job is scheduled first; if with the second machine, the job is
scheduled last.
 3. Once a job is scheduled, eliminate it.
 Apply steps 2-3 to the remaining jobs until all jobs are
assigned. The process will move toward the center of the
sequence.
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
Number of Servers
Single Server
Multiple Servers
Examples:
Multiple Single Servers
Examples:
Queue Discipline
First come -- first served (FCFS):
Multiple Priorities:
Examples:
Project Management
• A project is a set of activities aimed at
meeting a goal, with a defined beginning
and end.
8-27
Project Management with Certain
Time Estimates
• Summary of steps:
–Determine activities that need to be
accomplished
–Determine precedence relationships and
completion times
–Construct network diagram
–Determine the critical path
–Determine early start and late start
schedules
8-28
Project Scheduling When the
Times of Activities are Uncertain
• Summary of steps:
– Determine the activities that need to be accomplished
– Determine the precedence relationships and completion
times
– Construct the network diagram
– Determine the critical path
– Determine the early start and late start schedules
– Calculate the variances for the activity times
– Calculate the probability of completing by the desired due
date
8-29
Project Scheduling with Time
Uncertainty
• The “Heuristic” approach to dealing with
timing uncertainty
–Based on understanding of individual activities
as conforming to a “beta” distribution
• Take three time estimates
–Optimistic - What is the (realistic) fastest we can
get an activity done?
–Pessimistic - What is the (realistic) worst case
scenario for delay?
–Most likely - What is our “most likely” estimate?
8-30
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
Best Methods among Priority
Rule
• SPT and EDD are superior in all measure.
SPT is generally the best for minimizing
flow and average number of jobs.
Critical Ratio (CR)
Ratio of time remaining to work time
remaining
Time
remaining
CR =
Work days remaining
= Due date - Today' s date
Work (lead ) time remaining
• Process job with smallest CR first
• Performs well on average lateness
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
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.
• Minimizes total idle time
• Several conditions must be satisfied
Johnson’s Rule Conditions
• Job time must be known and constant
• Job times must be independent of
sequence
• Jobs must follow same two-step
sequence
• Job priorities cannot be used
• All units must be completed at the first
work center before moving to second
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
Queuing Theory
Where is waiting?
Service facility
 Fast-food restaurants
 post office
 grocery store
 bank
Manufacturing
Equipment awaiting repair
Phone or computer network
Product orders
Measures of System Performance
•
Average number of customers waiting
• Average time customers wait
• System utilization
Application of Queuing Theory
We can use the results from queuing theory to make the
following types of decisions:
 How many servers to employ
 Whether to use a single fast server or a number of
slower servers
 Whether to have general purpose or faster specific
servers
Goal:
Minimize total cost = cost of servers + cost of waiting
System Characteristics
•
Number of servers
• Arrival and service pattern
 Infinite Population – The arrival is not
affected by waiting line (public places,
e.g. gas station)
 Finite Population – The arrival is affect by
waiting line.
• Queue discipline