Mb0044 Unit 13 Slm

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Production and Operations Management Unit 13

Sikkim Manipal University Page No. 309

Unit 13 Operations Scheduling

Structure:

13.1 Introduction

Objectives

13.2 Purpose of Operations Scheduling

13.3 Factors Considered while Scheduling

13.4 Scheduling Activity under PPC

13.5 Scheduling Strategies

Detailed scheduling

Cumulative scheduling

Cumulative-detailed combination

Priority decision rules

13.6 Scheduling Guidelines

13.7 Approaches to Scheduling

Forward scheduling

Backward scheduling

13.8 Scheduling Methodology [Quantitative]

Charts and boards

Priority Decision Rules

13.9 Scheduling in Services

13.10 Summary

13.11 Glossary

13.12 Terminal Questions

13.13 Answers

13.1 Introduction

In the previous unit, we have dealt with the concepts of supply chain

management, domain applications, views on supply chain, bullwhip effect in

SCM, collaborative supply chain, inventory management in supply chain,

and financial supply chain. In this unit, we will deal with the purpose of

operations scheduling, factors considered while scheduling, scheduling

activity under PPC, scheduling strategies, scheduling guidelines,

approaches to scheduling, scheduling methodology, and scheduling in

services.

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Production is the transformation of inputs to the desired outputs. The

highest efficiency of the production is obtained by manufacturing therequired quantity, to the required quality, at the required time, and with the

best processes. To achieve these, the production management employs

Production Planning and Control department, known as PPC, whose

function is to coordinate all the production activities.

As the name of PPC represents, planning activity is the choice from several

alternatives of utilising the existing resources to achieve the desired

objectives and the control is monitoring of performance through the

feedback by comparing the results accomplished against the target and

taking corrective actions to improve the performance. Operations are the

processes performed on machines to achieve the desired results. Hence,PPC is a tool for directing all manufacturing operations, coordinating and

ensuring the end results of desired quantity, quality, time, and cost of

production. PPC functions are wide spread in planning and control of

materials, methods, machines, manpower, routing, estimating, scheduling,

dispatching, expediting, and evaluating the output.

Scheduling is the assignment of work to the production units with detailed

specifications of the times and the sequence of manufacturing until the end

product is rolled out and ready for dispatch. It also provides the performance

yardsticks on the time required to perform each operation and for the entire

series of activities as routed. Scheduling refers to firming up processing

times so that all the jobs/tasks are completed by the time they are due for

delivery to customers.

Objectives:

After studying this unit, you should be able to:

describe how scheduling aims at achieving the required output with

minimum of delay and disruption in processing

explain how it provides the means to maximise utilisation of men,

machines, and materials by maintaining free flow of materials along the

production lines and units identify how it helps to prevent the unbalanced allocation of time among

work centres with the view to eliminate idle capacity

recognise how it keeps the production cost to minimum

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13.4 Scheduling Activity under PPC

Table No 13.1: Terminologies

Routing Routing is the planning activity undertaken to determine the

best route for manufacturing a product. It lays down the flow

of work in the plant and determines what work is to be done,

where, and how. Routing considers plant layout, storage

space for inventory, and material handling. Route sheets

are developed for each job.

Scheduling Determines the order of processing jobs at each work

centre and establish their start and finish times (Discussed

in detail below)

Dispatch Allow production to commence through supply of materials

and work orders

Follow up Monitor progress and take corrective actions to minimise

deviations

13.5 Scheduling Strategies

Scheduling strategy differs from organisation to organisation as it depends

on the quantum of production, size and type of production, company’s

policy, priorities, etc. Most of these strategies are concerned with job shop

production since the problems encountered is more when more than one

product is produced in the same plant.

Following are the classifications:

Detailed scheduling

Cumulative scheduling

Cumulative-detailed scheduling

Priority decision rules

Let us now discuss these classifications in detail.

13.5.1 Detailed scheduling All job orders from customers are scheduled to the last details. This may not

be practical in case disruptions are there in production line like machine

breakdown, absenteeism, etc. (Possible in airlines, hotels, etc)

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13.5.2 Cumulative scheduling

The customer orders are pooled to form a cumulative work load and thenmatched with the capacity. The work load is then allocated in such a way

that immediate periods get allocated to maximum capacity.

13.5.3 Cumulative-detailed combination

This combines both the earlier strategies of firm and flexible nature of work

load. Cumulative work load projections can be used to plan for capacity as

needed. As changes happen during the week, the materials and capacity

requirements are updated. The actual time allocated to the specified job at

each work centre is as per the standard hours needed. This is tuned further

with the requirements of the master schedule.

13.5.4 Priority decision rules

When a set of orders are to be executed, the question of prioritising arises.

These priority decision rules are scheduling guidelines used independently

or in conjunction with any one of the above three strategies. These are

discussed later in section 13.8.2.

13.6 Scheduling Guidelines

1) Provide a realistic schedule

A schedule should be realistic rather than idealistic considering all the

practical possibilities.2) Allow adequate time for operations

Enough time should be allowed for production.

3) Allow adequate time before, between, and after operations

Enough time should be allowed to queuing and transit of Work in

Progress or FIG.

4) Don’t release all available jobs to the shop

Releasing all the available jobs as and when they are received

overloads the capacity. It also increases the lead time and excess work-

in-process inventory.

5) Don’t schedule all available capacity in the shop

Some excess capacity should be available to handle emergencies and

sudden alteration in jobs or to accommodate a totally new but profitable

order.

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6) Load only selected work centres

Only those work centres which are fit are selected for operations.7) Allow for necessary changes

Schedules should be open to necessary changes and alterations in the

products.

8) Gear up the entire shop to be responsible for the schedule

It is the worker’s responsibility to cooperate and meet the schedule.

13.7 Approaches to Scheduling

There are two types of approaches to scheduling. They are forward

scheduling and backward scheduling. These are used to ensure that the

lead time for manufacture is kept to a minimum and the products are

supplied to the customers as quickly as possible.

13.7.1 Forward scheduling

Forward scheduling is an approach where the customer orders are

immediately processed as soon as they come in even if their due dates are

far away. With forward scheduling, the scheduler selects a planned order

release date and schedules all the activities from this point forward and

ready within time.

13.7.2 Backward scheduling

With backward scheduling, the scheduler begins with a planned receipt dateor due date and moves backward in time according to the required

processing times. It is an approach where the customer orders are

processed as late as possible so that they are finished and delivered exactly

on their due dates. Here the starting time of the processing job is

determined by “setting back” the number of days required for its processing,

from finish date.

Illustration 1: Suppose all processes required for a component takes 20

days, then forward and backward scheduling can be executed as follows:

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Forward scheduling

End of operations Delivery due date

Work forward

Opns 01 02 03 04 05 06 07 08 (Finished Goods Inventory

Days: 2 5 6 9 11 13 15 17 (three days left for delivery)

Backward Scheduling:

Start of Operations (start late by three days) Delivery due date

End of operations

Operations 01 02 03 04 05 06 07 08 FGI

Days: 2 5 6 9 11 13 15 17

Work backward

Fig. 13.1: Forward and backward scheduling

13.8 Scheduling Methodology [Quantitative]

There are many types of scheduling and the methodologies used in

production. Specific methods used depend on the type of industry,

organisation, products, and level of sophistication in the production.

However the scheduling methodology can be classified as (i) charts and

boards and (ii) priority decision rules.

13.8.1 Charts and boards

a) Gantt load chart

b) Gantt progress chart

c) Schedule boards

d) Computer graphics

Let us now discuss these methodologies in brief.

a) Gantt load chart

A Gantt load chart shows the amount of cumulative workload that each work

centre has in a manufacturing unit. It is a graph showing individual and total

estimated workloads of each work centre on a time scale.

Uses of Gantt chart

Total workload shown graphically is simple, clear, and easy to

understand.

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It indicates the need for more resources or for reassigning or resources

when the load at one work centre becomes too large. In case some work centres are overloaded, employees from a low-load

work centre may be temporarily shifted to high load centres.

Limitations of Gantt chart

Gantt load chart does not account for delays and disruptions at the work

centres

It does not give information regarding the due date requirements of each

job

Chart must be updated periodically for new jobs Illustration 2

Work Centre Cumulative Work Loads

Sheet

Metal

A (5) B(16) C(14) D(9) E(11)

0 5 21 35 44 55

SheeringBending

A(4) B(12) D(24)

0 4 16 40

Painting A(5) B(7) C(12) D(4) E(6)

0 5 12 24 28 34

Brazing C(8) E(16)

0 8 16

Fig. 13.2: Work centres & the cumulative work load

In a production facility where five jobs to be processed through one or more

of the four work centres, the cumulative work load can be shown in the chart

given above (Fig 13.2). The work centre “Sheet Metal” has cumulative work

load of 55 days to complete all the five jobs (A to E). Similarly the

cumulative load for completion of three jobs in work centre ‘Shearing and

Bending’ is 40 days. The painting section having 5 jobs will take 34 days

and brazing section for only two jobs will have cumulative work load of 16

days. This chart will indicate only the cumulative workloads for each job and

to what extent the work centres are to be scheduled.

b) Gantt schedule and progress chart

A Gantt schedule and progress chart indicates the scheduled starting and

completion dates for each job as well as the current status of each job. The

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chart shows an immediate comparison between the schedule and reality

with respect to each job at work centres.Illustration 3

There are five machines with four jobs to be finished. The work schedule for

sheet metal work and soldering on job X is 4 and 4 days. The schedule for

braze on Y job is 5 days, Z job is scheduled for sheet metal wok for 6 days

and a finishing job of soldering a job F is of 3 days. (See Fig 13.3)

Job Machine 1 2 3 4 5 6 7 8 9 10 11

X(4) Sheet

work-1

X(4) Solder

Y(5) Braze

Z(6) Sheet

work-2

F(3) Solder

Fig. 13.3: Progress chart

Jobs are shown in Column one and the work completed on a particular days

in the progress chart above, is shown As against 4 days of sheet metal work-1 of X component, the operation has

completed up to 3.5 days, and hence there is half a day work pending on

that day.

a) Job X requires half day of change over on 5th day-shown as Black

square and then, soldering operation is continued for next four days.

b) Job Y Brazing continued for five days and as on this day it is as per

schedule, i.e., it has taken 5 days to complete as scheduled.

c) Since component X requires half day for completion before component

Z starts its operation on the sheet metal machine, six days required for

this Z will take completion date to 10.5 days.

d) Soldering operation on component F has already taken half a day extra

for completion (3.5 days instead of 3 days).

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This is how the progress charts shows the present day status of the job on

hand, jobs completion, jobs within the schedules, and jobs outside theschedule.

c) Schedule boards

Shop floor personnel need to know the information as to how they are

processing components in production and this can be reflected in a simple

schedule board inside the production shops. Content of these boards are

updated daily by the progress department. The boards contain simple bar

graphs to represent the actual status of components/products.

d) Computer graphics

Computer graphics and reports have replaced the mechanical boards and

charts. With the help of computers, PPC keep track of hundreds of itemsand can revise the schedules.

13.8.2 Priority decision rules

A priority decision rules shown below are the systematic procedures for

assigning priorities to waiting jobs, and determining the sequence in which

jobs are required to be processed.

The major criteria for applying rules are set up costs, idle time of machine

and labor, in-process inventory, percentage of jobs that are late, average

number of jobs waiting in queue, average time to complete job, and

standard deviation of time to complete job.Classifications of priority decision rules

A. Single-criteria rules

B. Combined criteria rules (Johnson’s rule)

C. Critical ratio scheduling

D. Index method of scheduling

E. Critical path method

Let us now discuss these classifications in detail.

A) Single-criteria rules

Here the jobs are assigned to the production division by considering singleand important criteria. These criteria’s are:

First come first served (the job that comes first is served by scheduling

first)

Earliest due date (here the job with the earliest due date is processed)

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Least slack available for production (here priority is given to the waiting

job whose slack time is least. Slack time is calculated as the differenceof the length of time remaining until the job is due and the length of its

operation time. For example, if a job requires 6 days and time left is 8

days, then the slack for that job is 2 days.)

Shortest processing time (job requires least of shortest time is

processed first)

Longest processing time (job that requires the longest time is processed

first)

Preferred customer order (priority to orders coming from favorite

customers)

Random selection (Jobs are selected at random and purely a chance forany job)

Illustration - 4

The illustration below details how the above said priority rules are applied

while scheduling. It also explains the different methods available for PPC

department.

Five jobs are to be processed at a Fabricating unit. The processing time in

days and the due date in days are given below.

Table 13.1: Data for illustration 4

Job A B C D E

Processing time in days 8 21 18 13 15

Due date in days 10 24 22 16 16

Determine:

a) The sequence of job according to the shortest processing time

b) Calculate total completion time

c) Calculate mean flow time or average completion time

d) Calculate average number of jobs in the system each day

e) Calculate average lateness

Solution

a) The sequence as is ABCDE, where as the sequence with the shortest

flow time is ADECB

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Table 13.2: Computation of flow time days and late days

JobSequence ProcessingTime in days Total Flowtime days Due daysfrom now Late(days)

A 8 8 10 -2

B 21 29 (8+21) 24 5

C 18 47 (29+18) 22 25

D 13 60 (47+13) 16 44

E 15 75 (60+15) 16 59

Total 75 219

b) Total completion time = Total process days = 75 days

c) Average number of jobs running at a time = Total cumulative flow time insequence at a time/total process days required. Here when the first job

is processed other four jobs are waiting. Similarly when the first job is

delivered, the second job is processed and the other three jobs are

waiting. This type of elimination goes ahead until all the five jobs are

completed.

Therefore, the average number of jobs in the system each day:

Avg No. of Jobs = [ (5x8) + (4x21) + (3x18) + (1x15) ] / (75)

= 2.92 jobs in the system/day

d) Sequencing jobs using short processing time (SPT)

This can be can be evolved by rearranging the above table as

ascending order of the processing time. This ascending order will be

ADECB, as shown below:

Table 13.3: Computation of flow time days and due days

Waiting Job -SPT Processingtime

Flow time indays

Due Date

A 8 8 10

D 21 29 (8+21) 24

E 18 47 (29+18) 22

C 13 60 (47+13) 16

B 15 75 (60+15) 16

Total 75 219

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Average/Mean Flow time = Total cumulative flow time in

days/number of sequences= 219/5

= 43.8 days.

This means the average job in the production line is 43.8 days.

e) Average job lateness = Cumulative late in days/no of sequences

= [0+5+25+44+59] / 5 = 26.6 days.

Here for A the lateness is (-2), i.e., produced 2 days earlier, hence

lateness is zero.

B) Combined criteria rules (Johnson’s rule)

Johnson’s rule is used to determine the sequence of order for a series of

jobs to be processed on a fixed number of machines. The basis for the

sequencing is that the total time required to complete all the jobs should be

minimum thereby reducing the idle time of all machines. Johnson’s rule is a

procedure that minimises the total cycle time in scheduling a group of jobs

on two workstations and the sequence of the jobs at the two work stations

should be identical and hence priority assigned to a job should be the same

at both. This type of production sequence for a group of jobs to minimise the

time has two advantages:

1) The group of jobs is completed in minimum time

2) Utilisation of two station flow shop is maximised

Steps used to adopt Johnson’s rule

Step - 1: List the processing/operation times from ‘n’ jobs on the two

work centres/machines.

Step - 2: Scan all the processing/operation times for the ‘n’ jobs on both

machines and select the shortest processing/operation time in

either work centre.

Step - 3: If the shortest processing/operation time happens to be for a

job on the first work centre/machine, place that job first in the

sequence. If it is at the second work centre/machine, then

place the job last in the sequence.

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Step - 4: Remove the job assigned to the sequence in step 3 from

further consideration (i.e. cross off both times for the assigned job).

Step - 5: Repeat steps 2, 3, and 4 until all the jobs are assigned into the

sequence.

Note: In case of a tie (i.e. a job having the same operation time on both the

machines or two jobs having the same operation time on either of the

machines), choose the job with the smallest subscript first for assigning into

sequence (i.e. when job A and job B has the same time on either machine

M1 and M2, choose job A first as compared to job B). On the other hand if

any job has the same processing/operation time on machines M1 and M2,

then give preference to the processing time/operation time on M1 to be

considered first for assigning that job into the sequence. The above rules

are known as ‘Thumb Rule’ to be followed in case of tie between two jobs or

between two machines.

Illustration - 5

Two machines working for six jobs to be produced whose time for two

operations are given below. Sequence them for scheduling by adopting

Johnson’s rule and find the minimum elapsed time and idle time on each

machine.

Table 13.4: Data for illustration 5Job Machine A

Time in hours

Machine BTime in hrs

1 7 12

2 16 14

3 9 13

4 6 10

5 13 7

6 15 5

Applying the Johnson’s rule for the above machines: 1) Among all the processing time, irrespective of which machine, the

shortest processing time is selected. In the given table, the shortest time

is for job 6 on machine B, and hence place this job as late as possible,

i.e., last.

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Illustration - 6

In the following table, four jobs are shown with its operation time, due dates,number of operations remaining, and shop remaining time. Calculate the CR

sequence schedule.


Job Operatingtime (hrs)

Timeremainingdue days

Number ofoperationsremaining

Shop timeremaining

AnswerCR

1 5 14 8 9 1.55

2 10 11 2 10 1.1

3 8 20 10 17 1.176

4 14 8 6 11 0.72

Answer:

Using CR to schedule, CR = [time remaining to due date]/shop time

remaining CR for first reading =14/9 = 1.55. Similar calculation and readings

are filled in the CR column. Arranging sequence is the lowest CR first and

then ascending order and hence sequence of job loading is 4, 2, 3, 1.

D) Index method of scheduling

This assigns job to the best machine until its capacity is exhausted and then

remaining jobs are assigned to the next best machine, etc. Here, the jobs

are assigned to the best work centre till it is fully loaded to capacity and the

remaining jobs are assigned to the next best if processing time is thecriterion. If the jobs can be processed in different work centres, “indices” are

calculated for the different likely process time with the lowest index time of

1.0.

Illustration - 7

Solve the following shop loading problem by using index method.


Job WC-1 WC-2 WC-3 WC-4

A 10 9 8 12

B 3 4 5 2

C 25 20 14 16

D 7 9 10 9

E 18 14 16 25

No. of days available 20 20 20 20

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The following table 13.6 shows the indices. The days corresponding to a job

assigned is with bold letter and underlined. Bold number and underlined isthe minimum number of days in each centre.

Table 13.7: Indices for illustration 7

WC1 WC2 WC3 WC4

Days Index Days Index Days Index Days Index

A 10 1.25 9 1.13 8 1.00 12 1.5

B 3 1.5 4 2.00 5 2.5 2 1.00

C 25 1.78 20 1.42 14 1.00 16 1.14

D 7 1.00 9 1.28 10 1.42 9 1.28

E 18 1.28 14 1.00 16 1.14 25 1.78

No ofDays

20 20 20 20

Daysassign

7 14 22 2

Indices are decided as: Job A has a minimum processing time of 8 days at

centre 3, and hence, this index is 1.00, i. e., the processing times of 10, 9,

and 12 at centres at 1, 2, and 4 are divided by 8 to find out the indices. This

procedure is followed for all jobs and their indices are indicated in the

column.

Assignment of the jobs to work centres based on the index equal to 1 is asfollows:

Table 13.8: Assignment of the jobs to work centres for illustration 7

Jobs A B C D E

Work centre assigned 1 4 3 1 2

[Additional Job of C can be assigned to work centre 4 as sufficient time

available]

E) Critical path method

It is used for scheduling large and unique projects in which the relationship

between the activities is quite intricate. The method overcomes thedeficiencies of Gantt chart.

Here a network of work centres and processing routes of each job is drawn

graphically. PERT/CPM charts are made to identify the critical path.

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Appointments

Here the customer is specified with the time of services. The advantage isthat the service is customised and utilisation of the capacity will be high. The

individual customer needs are satisfied. Service activities are determined

and planned for the customer. For example, surgery in hospitals.

Self Assessment Questions

1. Which of the following is not a characteristic of forward scheduling?

a) Production starts after the job order is received

b) Start and finish time is found by the earliest time slot available at a

work centre

c) Jobs starts in advance as per the latest available time slot in workcentre

2. In which of the following, the Gantt chart application is not there?

a) Used to track performance of shop floor employees

b) Used to reflect work load levels for machines and workstations

c) Can adopt change in scheduling each work centre

3. While job sequencing, _________ the critical ratio, __________ the

priority is given.

a) higher, higher

b) higher, lower

c) lower, higher4. Which of the following is not a priority rule?

a) Shortest processing time rule

b) Allow longest slack time

b) Earliest due date processing rule

c) Critical ratio rule

5. Which one of these is not a scheduling activity?

a) Dispatch

b) Routing

c) Scheduling

d) Facility planning6. Fill up the blanks with appropriate word/words

a) A priority decision rule is a systematic procedure for _________

_________ to waiting jobs, and determining the sequence in which

jobs are required to be processed.

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b) __________ rule is used to determine the sequence of order for a

series of jobs to be processed on a fixed number of machines.c) Critical ratio = [Time remaining for due date] / [ ________________ ]

13.10 Summary

Let us summarise the key learnings of this unit:

PPC focus on operation control through the process of scheduling.

Scheduling helps in operational controls in manufacturing and service

systems. The factors that greatly influence scheduling are the number of

jobs, the number of machines as well as the complexity of the machining

operations in the line.

There are two types of approaches to scheduling. They are forward

scheduling and backward scheduling. These are used to ensure that the

lead time for manufacture is kept to a minimum and the products are

supplied to the customers as quickly as possible.

The scheduling methodology adopted as well as its complexity differ

from production shop to shop and also in the systems like job shops,

process shops, mass production lines, fully automated plants, etc. There

are many types of scheduling and the methodologies used in production.

Specific methods used depend on the type of industry, organisation,

products, and level of sophistication required in the production.

The scheduling methodology can be classified as charts and boards

(Gantt load and progress chart, scheduled boards, and computer

graphics) and priority decision rules like single and double criteria rules,

critical ratio scheduling, index method, and critical path method. Each

one of these has its own specific applications and PPC will decide based

on the type of scheduling required to accomplish the job with the

minimum time possible, thus saving costly machine hours.

Workforce scheduling reflects the staffing plan into a specific work

schedule for each employee. Scheduling should ensure proper loading

of jobs based on the skills available within the workforce.

13.11 Glossary

Critical path: The critical path is the path which has the largest amount of

time associated with the activities and this represents the minimum time

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required to be considered while scheduling. This critical path time, which is

the minimum required time, has to be included while calculating the leadtime for delivery of products.

Queuing system: Just like queuing for a railway ticket in front of the

counter, the customer waiting in line for the supplies to be made is

considered while preparing an optimal scheduling model in order to ensure

minimum queuing of the customers.

13.12 Terminal Questions

1. What are the factors to be considered by PPC while scheduling?

2. Explain briefly the four classification of scheduling strategies?

3. Distinguish between forward scheduling and backward scheduling?

4. Explain in brief the uses and limitations of Gantt charts used in

Scheduling?

5. Distinguish between Gantt load charts and Gantt scheduling chart?

6. What is priority sequencing and what are the various criteria used in it?

13.13 Answers

Self Assessment Questions

1. (c)

2. (a)

3. (c)

4. (b)

5. (d)

6. a. Assigning priorities

b. Johnson’s

c. Time needed to completed the job

Terminal Questions

1. Refer 13.3

2. Refer 13.53. Refer 13.7

4. Refer 13.8.1

5. Refer 13.8.1

6. Refer 13.8.2

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Reference:

Frazier, G., & Gaither, N. (2002). Operations Management. South-Western/Thomson Learning.

Ronald, E. J., & Everett, A. E. (2009). Production And Operations

Management: Concepts, Models And Behavior. Phi Learning

E-Reference:

www.enotes.com

www.som.umass.edu

http://www.enotes.com/

http://www.enotes.com/

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