Priority Rules

• Possible priority Rules

• First come First Served (FCFS) :Jobs are processed in the order in which they arrive at a machine or work center.

• Shortest Processing time (SPT) : Jobs are processed according to processing time at a machine or work center, shortest job first.

• Earliest Due Date (EDD) : Jobs are processed according to due date

• Critical Ratio : Jobs are processed according to the smallest ratio of time remaining until the due date to processing time remaining.

• Slack per operation (S/O) : Jobs are processed according to the average slack time (time until due date minus remaining time to process).

• Rush :Emergency or preferred customer first.

• Priority rules can be classified as either local or global

• Local rules take into account information pertaining only to a single workstation.

• FCFS,SPT,EDD are local rules.

• Global rules take into account information pertaining to multiple workstations.

• Effectiveness of any given sequence is judged using the following performance measures:

• Jobflowtime: length of time a job is at a particular workstation or work center.ie length of time that begins when a job arrive at the workstation and ends when it leaves the workstation.

• It includes not only the actual processing time but also any waiting time to processed,transportation time between operations.

• Job lateness :length of time the job completion date is expected to exceed the date the date the job was due or promised to a customer.

• It is the difference between the actual completion time and the due date.

• Make span :total time needed to complete a group of jobs from the beginning of the first job to the completion of the last job.

• Average number of jobs: Jobs that are in a shop are considered to work in process inventory

• Average number of jobs = total flow time / Make span