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Operations Management: Sustainability and Supply Chain Management

Third Canadian Edition

Chapter 15

Short-Term Scheduling

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Outline (1 of 2)

Global Company Profile: Air Canada

The Importance of Short-Term Scheduling

Scheduling Issues

Scheduling Process-Focused Facilities

Loading Jobs

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Outline (2 of 2)

Sequencing Jobs

Finite Capacity Scheduling (FCS)

Scheduling Repetitive Facilities

Scheduling Services

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Learning Objectives (1 of 2)

When you complete this chapter you should be able to:

Explain the relationship between short-term scheduling, capacity planning, aggregate planning, and a master schedule

Draw Gantt loading and scheduling charts

Apply the assignment method for loading jobs

Name and describe each of the priority sequencing rules

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Learning Objectives (2 of 2)

When you complete this chapter you should be able to:

Use Johnson’s rule

Define finite capacity scheduling

Use the cyclical scheduling technique

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Air Canada

About 10% of Air Canada’s flights are disrupted per year, half because of weather

Cost is millions of dollars in lost revenue, overtime pay, food and lodging vouchers

The Operations Control Centre at Toronto’s Pearson Airport adjusts to changes, notifies passengers, and keeps flights flowing

Labour disruptions also causes delays

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We’ve all been stuck at the airport at one time or another. Air Canada and other airlines have worked hard to make such waits less frequent and less painful. Airline scheduling has been called “the Mother of all scheduling problems,” because the companies must schedule thousands of planes with interconnections daily, plus pilots and flight crews with all of their human resource constraints, plus local staff at each airport. This application is where mathematical programming really shines. The static scheduling problem is difficult enough, but the dynamic scheduling problem kicks in when a plane breaks down or Mother Nature gets nasty. One delayed plane can cause a ripple effect throughout the system. The interesting Global Company Profile in Chapter 15 describes how Air Canada handles the 10% of flights that are disrupted each year. The company has created a central control centre to anticipate and react to potential problems. The estimated annual savings are in millions of dollars paid for the “high-tech nerve centre” in just one year.

Short-Term Scheduling

Short-term schedules translate capacity decisions, aggregate planning, and master schedules into job sequences and specific assignments of personnel, materials, and machinery

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This slide defines short-term scheduling, the focus of Chapter 15. After all of the higher-level planning that the book has discussed, we finally see how to determine exactly what is produced and when.

Importance of Short-Term Scheduling

Effective and efficient scheduling can be a competitive advantage

Faster movement of goods through a facility means better use of assets and lower costs

Additional capacity resulting from faster throughput improves customer service through faster delivery

Good schedules result in more dependable deliveries

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Good scheduling increases utilization rates on equipment, while producing items quicker and on-time more often.

Scheduling Issues

Scheduling deals with the timing of operations

The task is the allocation and prioritization of demand

Significant issues are

The type of scheduling, forward or backward

The criteria for priorities

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Companies schedule jobs (products or customers), resources (machines, rooms, and employees), and activities (maintenance, purchasing, etc.). The resulting schedules depend upon the priorities that management has deemed important. Slide 10 (Table 15.1) provides examples of scheduling activities at five different organizations.

LO 1: Explain the relationship between short-term scheduling, capacity planning, aggregate planning, and a master schedule.

Scheduling Decisions

Table 15.1 Scheduling Decisions

Organization	Managers Schedule the Following:
The Hospital for Sick Children	Operating room use Patient admissions Nursing, security, maintenance staff Outpatient treatments
University of Manitoba	Classrooms and audiovisual equipment Student and instructor schedules Graduate and undergraduate courses
Magna International factory	Production of goods Purchases of materials Workers
Hard Rock Cafe	Chef, waiters, bartenders Delivery of fresh foods Entertainers Opening of dining areas
Air Canada	Maintenance of aircraft Departure timetables Flight crews, catering, gate, and ticketing personnel

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Scheduling Flow

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By this point in the book, students have been exposed to an array of planning functions at different levels of aggregation. This slide (Figure 15.1) does a nice job of showing where short-term scheduling fits in with the other planning functions

Forward and Backward Scheduling (1 of 3)

Forward scheduling starts as soon as the requirements are known

Produces a feasible schedule though it may not meet due dates

Frequently results in buildup of work-in-process inventory

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Scheduling involves assigning due dates to specific jobs, but many jobs compete simultaneously for the same resources. These slides describe forward vs. backward scheduling. Forward scheduling (Slide 12) may be more appropriate in situations where customer orders arrive and the goal is to complete each order as soon as possible. Here due dates are not considered, causing some jobs to potentially be late. Backward scheduling (Slide 13), which may be more appropriate for repetitive manufacturing environments or for services with specific event times (a concert or a doctor appointment), attempts to produce a schedule that meets all due dates. However, the schedule may not be feasible given the available resources. Slide 14 informs us that the two scheduling approaches are often combined in an attempt to address the trade-off between schedules that are feasible given available resources and schedules that meet all customer due dates.

Forward and Backward Scheduling (3 of 3)

Backward scheduling begins with the due date and schedules the final operation first

Schedule is produced by working backwards though the processes

Resources may not be available to accomplish the schedule

Often these approaches are combined to develop a trade-off between a feasible schedule and customer due dates

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Slide 14 informs us that the two scheduling approaches are often combined in an attempt to address the trade-off between schedules that are feasible given available resources and schedules that meet all customer due dates.

Different Processes/Different Approaches

Table 15.2 Different Processes Suggest Different Approaches to Scheduling

Process-focused facilities	Forward-looking schedules MRP due dates Finite capacity scheduling
Work cells	Forward-looking schedules MRP due dates Detailed schedule done using work cell priority rules
Repetitive facilities	Forward-looking schedule with a balanced line Pull techniques for scheduling
Product-focused facilities	Forward-looking schedule with stable demand and fixed capacity Capacity, set-up, and run times known Capacity limited by long-term capital investment

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This slide is based on Table 15.2, which provides an overview of different processes and approaches to scheduling.

Scheduling Criteria

Minimize completion time

Maximize utilization of facilities

Minimize work-in-process (WIP) inventory

Minimize customer waiting time

Optimize the use of resources so that production objectives are met

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This slide identifies four important scheduling criteria. (Note that criteria 1 and 3 are often very highly correlated.) Other criteria may be used, such as minimizing the maximum lateness of any job, minimizing the average lateness for those jobs that are late, minimizing the number of late jobs, or some sort of fairness criterion for customers or employees. Determination of the most important criteria often drives which scheduling method to use.

Scheduling Process-Focused Facilities (1 of 3)

Schedule incoming orders without violating capacity constraints

Check availability of tools and materials before releasing an order

Establish due dates for each job and check progress

Check work in progress

Provide feedback

Provide work efficiency statistics and monitor times

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On a daily basis, job shops tend to be more difficult to schedule than assembly lines or continuous flow facilities due to the variety of products produced, different routes through the system that these products take, and unstable demand. (The product-focused facilities might be more likely to utilize mathematical programming to set a weekly or monthly schedule, but once in place, these schedules seldom need to be updated—certainly not on a daily basis.) Slide 17 identifies the features that a good production planning and control system should incorporate.

Scheduling Process-Focused Facilities (2 of 3)

Table 15.2 Different Processes Suggest Different Approaches to Scheduling

Process-focused facilities (job shops)

Focus is on generating a forward-looking schedule.

MRP generates due dates that are refined with finite capacity scheduling techniques.

Examples: foundries, machine shops, cabinet shops, print shops, many restaurants, and the fashion industry.

Work cells (focused facilities that process families of similar components)

Focus is on generating a forward-looking schedule.

MRP generates due dates, and subsequent detail scheduling/dispatching is done at the work cell with kanbans and priority rules.

Examples: work cells at ambulance manufacturer Wheeled Coach, aircraft engine rebuilder Standard Aero, greeting-card maker Hallmark.

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Scheduling Process-Focused Facilities (3 of 3)

Table 15.2 Continued

Repetitive facilities (assembly lines)

Focus is on generating a forward-looking schedule that is achieved by balancing the line with traditional assembly-line techniques.

Pull techniques, such as JIT and kanban, signal component scheduling to support the assembly line.

Challenging scheduling problems typically occur only when the process is new or when products or models change.

Examples: assembly lines for a wide variety of products from autos to home appliances and computers.

Product-focused facilities (continuous)

Focus is on generating a forward-looking schedule that can meet a reasonably stable demand with the existing fixed capacity.

Capacity in such facilities is usually limited by long-term capital investment.

Capacity is usually known, as is the setup and run time for the limited range of products.

Examples: facilities with very high volume production and limited-variety products such as paper on huge machines at International Paper, beer in a brewery at Labatt Brewing Company, or rolled steel in a Dofasco plant.

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Planning and Control Files

Planning Files

An item master file contains information about each component

A routing file indicates each component’s flow through the shop

A work-centre master file contains information about the work centre

Control Files

Track the actual progress made against the plan

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This slide identifies the types of planning and control files that help to ensure that a scheduling system is accurate and relevant.

Loading Jobs

Assign jobs so that costs, idle time, or completion time are minimized

Two forms of loading

Capacity oriented

Assigning specific jobs to work centres

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Loading means the assigning of jobs to work or processing centres. When loading is undertaken via the perspective of capacity, input-output control can be used. When loading is undertaken via assigning specific jobs to work centres, either Gantt charts or the assignment method of linear programming can be used.

Input-Output Control (1 of 2)

Identifies overloading and underloading conditions

Prompts managerial action to resolve scheduling problems

Can be maintained using ConWIP cards that control the scheduling of batches

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Slide 20: Input-output control compares arrival rates of new jobs to the capacity of the facility to identify overloading (too much work) or underloading (too much idleness) conditions. ConWIP (constant work-in-process) cards can be used to aid input-output control. Such cards control the amount of work in a work centre. Once the job has been completed, its ConWIP card is released and returned to the initial workstation, authorizing the entry of a new batch into the work centre. Note that the cumulative change in backlog must be computed as the sum of the actual inputs minus the sum of the actual outputs. When this value moves too far from 0, managers should consider actions such as those identified in Slide 21. Changing work centre input can be accomplished by (a) routing work to or from other work centres, (b) increasing or decreasing subcontracting, or (c) producing less (or more).

Input-Output Control (2 of 2)

Options available to operations personnel include:

Correcting performances

Increasing capacity

Increasing or reducing input to the work centre

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Note that the cumulative change in backlog must be computed as the sum of the actual inputs minus the sum of the actual outputs. When this value moves too far from 0, managers should consider actions such as those identified in Slide 21. Changing work centre input can be accomplished by (a) routing work to or from other work centres, (b) increasing or decreasing subcontracting, or (c) producing less (or more).

Gantt Charts

Load chart shows the loading and idle times of departments, machines, or facilities

Displays relative workloads over time

Schedule chart monitors jobs in process

All Gantt charts need to be updated frequently to account for changes

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Slide 22: Gantt charts are visual aids that are useful in loading and scheduling. The charts show the use of resources, such as work centres and labour. A Gantt load chart shows the relative workloads over time of departments, machines, or facilities. Managers can shift work between resources when overloading or underloading appears. A Gantt schedule chart shows jobs in process, indicating which are on schedule and which are behind or ahead of schedule. Again, this type of chart provides management with a quick visual aid to look for areas requiring action. Slide 23 (Example 2) provides an example of a load chart (here metalworks and painting are completely loaded for the entire week), and Slide 24 (Example 3) provides an example of a schedule chart (here job A is one-half day behind schedule, job B was completed on time, and job C is ahead of schedule).

LO2: Draw Gantt loading and scheduling charts.

Gantt Load Chart Example

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Slide 23 (Example 2) provides an example of a load chart (here metalworks and painting are completely loaded for the entire week).

Gantt Schedule Chart Example

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This slide (Example 3) provides an example of a schedule chart (here job A is one-half day behind schedule, job B was completed on time, and job C is ahead of schedule).

Sequencing Jobs

Specifies the order in which jobs should be performed at work centres

Priority rules are used to dispatch or sequence jobs

FCFS: First come, first served

SPT: Shortest processing time

EDD: Earliest due date

LPT: Longest processing time

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LO 4: Name and describe each of the priority sequencing rules.

Once jobs are loaded, managers must decide the sequence in which they are to be completed (also called dispatching). Students regularly perform sequencing in their own lives when they decide in what order to work on their assignments or which of several errands to run first. This slide identifies the four most popular priority rules for sequencing. A nice feature of sequencing is that, before any processing begins, different priority rules can be easily tested and the results compared. In fact, the rule used might change from one application to another depending on comparison results.

When comparing sequencing rules the four main priority rules have to be applied to the same set of jobs waiting to be sequenced. Note that flow time is defined as total time in the system (waiting plus processing). Job lateness is computed as Max (0, flow time – due date). (Here we assume no penalties for finishing early.) Clearly the lateness depends upon the sequence chosen. While total processing time will not depend upon the sequence, total flow time will because the flow time of any particular job in the sequence becomes part of the waiting time for all other jobs that follow. (This is why SPT is the rule that minimizes total flow time—overall waiting is minimized by getting the quick jobs finished right away.) Four measures of effectiveness can be computed: (1) average completion time, (2) utilization metric, (3) average number of jobs in the system, and (4) average job lateness. Note that the first three measures are completely correlated, i.e., the rule that performs best on one will perform best on all three.

Comparison of Sequencing Rules

No one sequencing rule excels on all criteria

SPT does well on minimizing flow time and number of jobs in the system

But SPT moves long jobs to the end which may result in dissatisfied customers

FCFS does not do especially well (or poorly) on any criteria but is perceived as fair by customers

EDD minimizes maximum lateness

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This slide provides general comparisons for the four common sequencing rules. While SPT does not always minimize average lateness, it does always minimize average completion time. A suggestion: “When in doubt, use SPT.” FCFS usually performs at a decent average level, and it has the prime advantage of being fair to customers. FCFS may be particularly appropriate when customers can see the queue of jobs (think about sitting in a restaurant and then watching a group who just entered get seated before you, or think about potential unfairness that arises with multiple grocery store lines). It is very important to emphasize that while EDD does minimize maximum lateness, it does not necessarily minimize average lateness or even number of late jobs. In fact, examples can be presented showing that EDD may perform very poorly on those lateness criteria. The basic idea is that if everything is scheduled via EDD, once the system falls behind, it may well remain behind for every single job that follows. This phenomenon is counter-intuitive and is one reason why students run into trouble meeting due dates during their semesters.

Critical Ratio (CR)

An index number found by dividing the time remaining until the due date by the work time remaining on the job

Jobs with low critical ratios are scheduled ahead of jobs with higher critical ratios

Performs well on average job lateness criteria

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If average lateness is the most important criterion, the critical ratio technique may the best one to use. It’s in some sense a hybrid of EDD and LPT that examines the slack available for each job. Slide 35 provides the formula. A primary advantage of this technique is that the ratios can be recomputed after each job finishes, which potentially changes the remaining sequence after the slack conditions have changed. Slide 36 identifies benefits of the critical ratio technique in most production systems.

Critical Ratio Technique

Helps determine the status of specific jobs

Establishes relative priorities among jobs on a common basis

Relates both stock and make-to-order jobs on a common basis

Adjusts priorities automatically for changes in both demand and job progress

Dynamically tracks job progress

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Slide 36 identifies benefits of the critical ratio technique in most production systems.

Limitations of Rule-Based Dispatching Systems

Scheduling is dynamic and rules need to be revised to adjust to changes

Rules do not look upstream or downstream

Rules do not look beyond due dates

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This slide identifies three limitations of using myopic rule-based dispatching systems. Nevertheless, schedulers often apply one of these methods at each work centre and then modify the sequence to deal with a multitude of real-world variables. They may do this manually or with finite capacity scheduling software.

Finite Capacity Scheduling (1 of 2)

Overcomes disadvantages of rule-based systems by providing an interactive, computer-based graphical system

May include rules and expert systems or simulation to allow real-time response to system changes

Initial data often from an MRP system

FCS allows the balancing of delivery needs and efficiency

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Finite capacity scheduling provides the scheduler with interactive computing and graphic output to help overcome the disadvantages of rule-based systems. FCS systems may allow virtually instantaneous schedule changes based on the most up-to-date information. Slide 45 (Figure 15.5) shows the different components that are combined to produce the Gantt chart output in FCS.

LO 6: Define finite capacity scheduling.

Finite Capacity Scheduling (2 of 2)

Figure 15.5

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Scheduling Repetitive Facilities (1 of 2)

Level material use can help repetitive facilities

Better satisfy customer demand

Lower inventory investment

Reduce batch size

Better utilize equipment and facilities

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Level material use refers to the use of frequent, high-quality, small lot sizes that contribute to just-in-time production. These slides identify advantages to repetitive producers of implementing level material use.

Scheduling Repetitive Facilities (2 of 2)

Advantages include:

Lower inventory levels

Faster product throughput

Improved component quality

Reduced floor-space requirements

Improved communications

Smoother production process

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Scheduling Services (1 of 3)

Service systems differ from manufacturing

Manufacturing	Services
Schedules machines and materials	Schedule staff
Inventories used to smooth demand	Seldom maintain inventories
Machine-intensive and demand may be smooth	Labour-intensive and demand may be variable
Scheduling may be bound by union contracts	Legal issues may constrain flexible scheduling
Few social or behavioural issues	Social and behavioural issues may be quite important

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This slide presents several ways in which scheduling service systems differs from scheduling manufacturing systems. Some of these service characteristics appear in the Hard Rock Café video case associated with this chapter.

Scheduling Services (2 of 3)

Hospitals have complex scheduling system to handle complex processes and material requirements

Banks use a cross-trained and flexible workforce and part-time workers

Retail stores use scheduling optimization systems that track sales, transactions, and customer traffic to create work schedules in less time and with improved customer satisfaction

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The complexity of scheduling services is exemplified by the industries highlighted in these slides.

Scheduling Services (3 of 3)

Airlines must meet complex FAA and union regulations and often use linear programming to develop optimal schedules

24/7 operations like police/fire departments, emergency hot lines, and mail order businesses use flexible workers and variable schedules, often created using computerized systems

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Demand Management

Appointment or reservation systems

FCFS sequencing rules

Discounts or other promotional schemes

When demand management is not feasible, managing capacity through staffing flexibility may be used

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As seen in Chapter 13, demand management techniques can be employed to smooth production, in this case easing the burden on the service company workforce scheduler.

Scheduling Service Employees With Cyclical Scheduling

Objective is to meet staffing requirements with the minimum number of workers

Schedules need to be smooth and keep personnel happy

Many techniques exist from simple algorithms to complex linear programming solutions

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Cyclical scheduling is utilized to develop a schedule with the minimum number of workers. Each employee is assigned to a shift and has time off. The steps used to address the staffing problem is described in Example 8 in the text.

LO 7: Use the cyclical scheduling technique.

Summary

Scheduling involves the timing of operations to achieve efficient movement of units through a system

Proper scheduling, loading, and sequencing of jobs all increase outputs

Many techniques are used, such as:

Gantt charts, assignment method of scheduling, various priority rules

Critical-ratio rule, Johnson’s rule, and finite capacity scheduling

Matching capacity to demand in service environments require different techniques

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Time remainingDue date–Today’s date

Workdays remainingWork (lead) time remai

ning