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Priority Rules
1.    What are Priority Rules
2.    Types of Priority Rules
3.    Example with Demo Solution1
4.    Example with Demo Solution2
5.    Simulation

1.    What are Priority Rules

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 isindependent 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 processs-focussed facilitiessuch as clinics, print shop and
manufacturing job shops. Priority Rules try to minimise completion time, number of jobs in the
system, and job lateness, while maximising facility utilisation.
The following standard measures of schedule performance are used to evaluate
Priority Rules:
Meeting due dates of customers or downstreamoperations.
Minimising the flow time(the time a job spends in the process).
Minimising work-in-process inventory.
Minimising idle time of machines or workers.

2.    Types of Priority Rules
The most popular PriorityRules are:
| FCFS (First Come First Serve)
--- The first job to arrive at a work centre is processed first. |

| EDD (Earliest Due Date)
--- The job with the earliest due date is selected first. |

| SPT (Shortest Processing Time)
--- The shortest job are handled first and completed. |

| LPT (Longest Processing Time)
--- The longer, bigger jobs are often very important and areselected first. |

| CR (Critical Ratio)
--- Critical Ratio is an index number computed by dividing the time remaining until due date
by the work time remaining. |
First Come First Serve (FCFS):

The first job to arrive at a work centre is processed first by using FCFS.
The steps for using this rule are:
| Firstly, the user will input the number of jobs, the jobnames,the processing time and the due date of each job or use the data values given at the starting point. |

| Secondly, calculate the flow time of each job by using the processing time. the flow time is the accummulations of processing time each job by each job. |

| Next, calculate the delays for each job of each job. |

| The delays is calculated from the flow time and due date. Theformula is:
Delay = Flow time - due date
However, there are two conditions for calculating the delays although apply the formula already:

If the delays calculated is less than flow time, then the delays =  zero.
If the delays calculated is more than flow time, then the delays =
the value of (flow time - due date). |

| The next step iscalculating the total processing time, total flow time and total delays from the accummulations processing time, the accummulations flow time and the accummulations delays. |

| Then, using the total flow time, total processing time, and total delays for calculating the average completion time, the utilisation, the average number of jobs in the system and the average job delays. |

| Theformulas for calculation are below:

Average completion time = sum of total flow / no. of jobs
Utilisation = total processing time / sum of total flow
Average no. of jobs  = total flow time / total processing time
Average job delays = total delays / no. of jobs |

| These above are the steps for using the FCFS priority rules. |
Demo Using FCFS in an Example:(Power Point...