1 OM2, Ch. 14 Operations Scheduling and Sequencing ©2010 Cengage Learning. All Rights Reserved. May...

1

OM2, Ch. 14 Operations Scheduling and Sequencing

©2010 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.

OPERATIONS SCHEDULING AND SEQUENCING

CHAPTER 14

DAVID A. COLLIERAND

JAMES R. EVANS

OM2

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LO1 Explain the concepts of scheduling and sequencing.

LO2 Describe staff scheduling and appointment system decisions.

LO3 Explain sequencing performance criteria and rules.

LO4 Describe how to solve single- and two-resource sequencing problems.

LO5 Explain the need for monitoring schedules using Gantt charts.

l e a r n i n g o u t c o m e s

Chapter 14 Learning Outcomes

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ean Rowecamp, clinical coordinator of nursing services, was faced with a deluge of complaints by her nursing staff about their work schedules and complaints by floor supervisors about inadequate staffing. The nurses complained they were having too many shift changes each month. Supervisors said they had too many nurses during the days and not enough at night and on the weekends. It seems that nothing she did would satisfy everyone. The nurses were unionized, so she couldn’t schedule them more than 7 consecutive working days and the nurses required at least 16 hours between shift changes. Nurses were constantly making “special requests” for personal time off, despite the negotiated procedures for bidding for shifts and vacation times. Jean lamented that she became an administrator and longed for the days when she was just a simple caregiver.

Chapter 14 Operations Scheduling and Sequencing

What do you think? As a student, how do you schedule your homework, school projects, and study activities? What criteria do you use?

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This chapter addresses key issues and methods for scheduling and sequencing in manufacturing and service organizations.

• Scheduling refers to the assignment of start and completion times to particular jobs, people, or equipment.

• Sequencing refers to determining the order in which jobs or tasks are processed.


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Telling Umpires Where to Go Some of the critical factors in developing major league umpire

schedules were to ensure that umpire crews were not assigned to consecutive series with the same team if possible; that the number of times a crew was assigned to a team was balanced over the course of the season; that travel sequences be rational and realistic; and that a variety of constraints be met. For instance, it makes more sense to schedule a crew to several consecutive series out on the East Coast or West Coast and move them to nearby cities rather than shuttle them back and forth across the country. Various constraints limited the scheduling possibilities. For example, one could not schedule a crew for a day game in another city after working a night game on the previous day. In addition, crews need time to rest and travel between game assignments. All these factors needed to be considered in the context of the game schedule, which was created well in advance.


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Scheduling and sequencing are fundamental to all three levels of aggregation and disaggregation planning (see Exhibit 13.1).

Level 3 decisions require detailed resource scheduling (trucks, labor, equipment, computers, and jobs), sequencing, and day-to-day execution.


Scheduling and Sequencing in the Resource Management Framework

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Scheduling in Supply Chains• The complexity of many business situations

dictates that effective scheduling systems be computerized.

• Scheduling and information exchange are at the heart of managing an efficient and responsive value chain because the network of processes needs to be synchronized.

• Computer-generated schedules and the sharing of production, purchasing, inventory, delivery, and customer information among suppliers and buyers in the value chain enable faster service at lower cost.


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Staff SchedulingStaff scheduling attempts to match available personnel with the needs of the organization by:

1. Accurately forecasting demand and translating it into the quantity and timing of work to be done

2. Determining the staffing required to perform the work by time period

3. Determining the personnel available and the full- and part-time mix

4. Matching capacity to demand requirements and developing a work schedule that maximizes service and minimizes costs


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Exhibit 14.1

T. R. Accounting Service is developing a workforce schedule for three weeks from now, and has forecast demand and translated it into the following minimum personnel requirements for the week.

Day Mon Tue Wed Thur Fri Sat SunMin Personnel 8 6 6 6 9 5 3

Staff Scheduling Procedure for T.R. Accounting Service

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Exhibit 14.2 Final Accountant Schedule

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Appointment SystemsFrom an operations’ perspective, appointments can be viewed as a reservation for service time and capacity.

Four decisions to make regarding designing an appointment system are:

1. Determine the appointment time intervals.

2. Determine the length of each workday and time off-duty.

3. Decide how to handle overbooking.

4. Develop customer appointment rules that maximize customer satisfaction.


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Appointment SystemsFrom an operations’ perspective, appointments can be viewed as a reservation for service time and capacity.

Given the perishable nature of professional service-provider time and the potential loss of revenue, most service providers overbook.

Example: If you book a dentist appointment and do not reschedule and do not show up, the dentist may lose forever the revenue he/she could make during that time.

Appointment systems are critical to maximizing revenue and minimizing idle time.


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Software to Schedule Anywhere

One provider of small business software offers an online employee scheduling system called Schedule-Anywhere (ScheduleAnywhere.com). This service allows managers to schedule employees from any computer with Internet access, whether at work, at home, or on the road. “With over 70,000 users, we get a lot of feedback on what people really need in an employee scheduling system,” said Jon Forknell, vice president and general manager of Atlas Business Solutions. “Many of our customers told us they needed an online solution that was affordable and easy to use.”


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ScheduleAnywhere gives users the power to • schedule employees from any computer with

Internet access• create schedules by position, department,

location, etc.• view schedule information in a 1-day, 7-day, 14-

day, or 28-day format• enter staffing requirements and view shift

coverage• see who’s scheduled and who’s available• automatically rotate or copy employee schedules• pre-schedule time-off requests• avoid scheduling conflicts• give employees read/write or read-only access to

schedules


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Sequencing

Sequencing is required when several activities must be processed using a common resource.

Example: An insurance claims analyst needs to process 25 customer medical claims on a computer (the resource). What claims should be processed first, second, and last to maximize customer satisfaction or minimize average claim lateness?


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Sequencing

Flow time is the amount of time a job spent in the shop or factory. Flow time is computed as follows:

Fi = ∑pij + ∑wij = Ci - Ri [14.1]


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SequencingMakespan is the time needed to process a given set of jobs. A short makespan aims to achieve high equipment utilization.

M = C - S [14.2]

where

M = makespan of a group of jobs

C = completion time of last job in the group

S = start time of first job in the group


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SequencingLateness and tardiness measure performance related to customer-focused due-date criteria.• Lateness is the difference between the completion

time and the due date (either positive or negative). • Tardiness is the amount of time by which the

completion time exceeds the due date. (Tardiness is defined as zero if the job is completed before the due date, and therefore no credit is given for completing a job early).

Li = Ci - Di [14.3]

Ti = Max (0, Li) [14.4]

where Li = lateness of job i

Ti = tardiness of job i

Di = due date of job i


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SequencingTwo of the most popular sequencing rules for prioritizing jobs are:

Shortest Processing Time (SPT)

With different processing times, SPT sequencing maximizes resource utilization and minimizes average flow time and work-in-process inventory.

Earliest Due Date (EDD)

Using Earliest Due Date (EDD), the maximum job tardiness and lateness are minimized.


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Single-Resource Sequencing Problem• In a serial manufacturing process, a bottleneck

workstation controls the output of the entire process. Therefore, it is critical to schedule it efficiently.

• With different processing times, SPT sequencing maximizes workstation utilization and minimizes average job flow time.

• When processing times are relatively equal, first-come-first-served sequencing is applied.

• Using Earliest Due Date (EDD), the maximum job tardiness and lateness are minimized.


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Exhibit 14.3 Comparison of Three Ways (By-the Numbers, SPT, and EDD)

to Sequence the Five Jobs

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Solved ProblemFive tax analysis jobs are waiting to be processed by Martha at T.R. Accounting Service. Use the shortest processing time (SPT) and earliest due date (EDD) sequencing rules to sequence the jobs. Compute the flow time, tardiness, and lateness for each job, and the average flow time, average tardiness, and average lateness for all jobs. Which rule do you recommend? Why?

Job Processing Time (days) Due Date1 7 112 3 103 5 84 2 55 6 17


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Solution

The SPT sequence is 4-2-3-5-1.

Due Lateness Tardiness

Job Flow (Fi) Date (Di) (Li) (Max (0, Li)

4 2 5 - 3 02 2 + 3 = 5 10 - 5 03 5 + 5 = 10 8 2 25 10 + 6 = 16 17 - 1 01 16 + 7 = 23 11 12 12

Average 11.2 + 1.0 2.8


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SolutionThe EDD sequence is 4-3-2-1-5.

Due Date Lateness TardinessJob Flow (Fi) (Di) (Li = Ci - Di) (Max (0, Li)

4 2 5 - 3 03 2 + 5 = 7 8 - 1 02 7 + 3 = 10 10 0 01 10 + 7 = 17 11 6 65 17 + 6 = 23 17 6 6Average 11.8 - 1.6 2.4Given the nature of the data, this is not an easy decision. The SPT rule minimizes average flow time and average lateness, but Job 5 is extremely late by 12 days. The EDD rule minimizes the maximum job tardiness and lateness. Jobs 1 and 5 are tardy by 6 days. If Job 5 is a big client with significant revenue potential, then the EDD rule is probably best.


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Two-Resource Sequencing Problem (often called Johnson’s Rule) • In the following example, we assume that each job

must be processed first on Resource #1 and then on Resource #2.

• Hirsch Products manufactures custom parts that first require a shearing operation (Resource #1) and then a punch-press operation (Resource #2). Order information is provided below.Job Shear (days) Punch (days)

1 4 5

2 4 1

3 10 4

4 6 10

5 2 3


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Exhibit 14.4

If jobs are completed by order number, the punch press often experiences idle time awaiting the next job as shown below (Exhibit 14.4). The makespan is 37 days.

Gantt Job Sequence Chart for Hirsch Product Sequence 1-2-3-4-5

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Johnson’s Rule results in a reduction in makespan from 37 days to 27 days, as shown in the Gantt chart below in Exhibit 14.5. So, smart scheduling is important for customer service and process efficiency!

Exhibit 14.5Gantt Job Sequence Chart for Hirsch Product Sequence 5-1-4-3-2

Using Johnson’s Rule

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Schedule Monitoring and Control• The scheduling process must be monitored on a

continuing basis to track changes in the status of orders, input materials, inventory changes, labor turnover, and sales commitments. Reschedules are a normal part of scheduling and sequencing.

• Short-term capacity fluctuations also necessitate changes in schedules and sequences.

• Gantt charts are useful tools for monitoring schedules. Exhibit 14.6 (next slide) shows a Gantt chart for a variety of jobs. This helps to track jobs that are behind, on, or ahead of schedule.


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Exhibit 14.6 Gantt Chart Example for Monitoring Schedule Progress

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Balloons Aloha Case Study

1. Compute the average flow time, lateness, and tardiness for this group of jobs using Mr. Sailboat’s sequential order of 1 (fi rst), 2, 3, 4, 5, and 6 (last).

2. In what order would the jobs be processed using the SPT rule? Compute the average flow time, lateness, and tardiness for this group of jobs.

3. Compare the answers in parts 1 and 2.4. What are your short-term recommendations for this

set of six jobs?Explain how you arrived at them.

5. What are your long-term recommendations with respect to sequencing jobs at Balloons Aloha? Explain your reasoning.


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