Priority Rules

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  • Topic: Delay, Scheduling, The Rules
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PRODUCTION AND OPERATION MANAGEMENT

TOPIC: PRIORITY RULES

NAME|I.D|
SHAMSIYYA UMAR MAGAM|9203167|

DEFINITION
Priority Rules provide guidelines for the sequence in which jobs should be worked. The rules generally involve the assumption that job setup cost and time is independent of processing times. In using this rules, job processing times and due dates are important pieces of information. Job times usually include setup and processing times. Due dates may be the result of delivery times promised to customers, MRP processing, or managerial decisions. The rules are especially applicable for process focused facilities such as clinics, print shop and manufacturing job shops. Priority Rules try to minimize completion time, number of jobs in the system, and job lateness (tardiness) while maximizing facility utilization.

USES
The following standard measures of schedule performance are used to evaluate Priority Rules:
Meeting due dates of customers or downstream operations
Minimizing the flow time (the time a job spends in the process)
Minimizing work-in-process inventory
Minimizing idle time of machines or workers

TYPES OF PRIORITY RULES
There are a lot of categories of priority rules, they are:
a). First come, First served
b). Shortest Operating Time
c). Earliest Due Date first
d). Earliest Start Date First
e). Least Slack Time Remaining (STR) first
f). Least Slack Time Remaining (per operating as opposed to per job - STR/OP) first
g). Smallest Critical Ratio First , (Due Date - Current Date) / (Number of days remaining)
h). Last Come, First Served
i). Random Order

The most popular Priority Rules are:
1. FCFS (First Come First Serve): The first job to arrive at work centre is processed first.
2. EDD (Earliest Due Date): The job with the earliest due date is selected first
3. SPT (Shortest Processing Time): The shortest job are handled first and completed.
4. LPT (Longest Processing Time): The longer, bigger jobs are often very important and are selected first.
5. CR (Critical Ratio): Critical Ratio is an index number computed by dividing the time remaining until due date by the work time remaining.

EXAMPLES WITH SOLUTIONS
Example 1.
There are five sheet metal jobs are waiting to be assigned at a company work centre. Their work (processing) times and due dates are given below. Then, determine the sequence of processing according to: a) FCFS, b) EDD, c) SPT, d) LPT rules.

JOB|PROCESSING TIME|DUE DATE|
A|6|8|
B|2|6|
C|8|18|
D|3|15|
E|9|2|

a) Solution using the FCFS rules:
The FCFS sequence is simply A-B-C-D-E. The "flow time" in the system for the sequence measures the time each job depends waiting plus being processed.

JOB SEQUENCE|PRCESSING TIME|FLOW TIME|DUE DATE|DELAYS|
A|6|6|8|0|
B|2|8|6|2|
C|8|16|18|0|
D|3|19|15|4|
E|9|28|23|5|
|28|77||11|

The First Come First Serve rule results in the following measures of effectiveness: Average completion time = total flow time / no. of jobs
= 77 days / 5
= 15.4 days

Utilization = total processing time / total processing time
= 28 days / 77 days
=36.4 %
Average no. of jobs = total flow time / total processing time = 77 days / 28 days
=2.75 jobs
Average job delay = total delay days / no. of jobs
= 11days / 5
= 2.2 days

b) Solution using the EDD rules:
The EDD rule gives the sequence B-A-D-C-E. Note that jobs are ordered earliest due date first.

JOB SEQUENCE|PROCESSING TIME|FLOW TIME|DUE DATE|DELAYS|
B|2|2|6|0|
A|6|8|8|0|
D|3|11|15|0|
C|8|19|18|1|
E|9|28|23|5|
|28|68||6|

The Earliest Due Date rule results in the following measures of effectiveness: Average completion time = total flow time / no. of jobs
= 68 days / 5
= 13.6 days
Utilization = total processing time / total flow time
= 28 days / 68 days
= 41.2%
Average no. of jobs = total flow time / total...
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