Priority Rules for Allocating Jobs to Machines
The process of determining which job is started first on a particular machine or work center is known as sequencing or priority sequencing. Priority rules are the criteria by which the sequence of job is determined. These can be very simple. requiring only that jobs be sequenced according to one piece of data. such as processing time, due date. or order of arrival. Other rules, though equally simple, may require several pieces of information.typically to derive an index number such as in the least slack rule and the critical ratio .
rule (both defined later). Still others, such as Johnson's rule (also discussed later), apply to job scheduling on a sequence of machines and require a computational procedure to specify the order of performance. Ten of the more common priority rules for sequencing jobs are.
I. FCFS-first come, first-served. Orders are run in the order that they arrive in the department.
2. SPT -shortest processing time. Run the job with the shortest completion time first next shortest second. and so on. This is identical to SOT-shortest operating time.
3. Due date-earliest due date first. Run the job with the earliest due date first.Date-when referring to the entire job; OPNDD-when referring to the next to perarion.
4. Start date-due date minus normal lead time. Run the job with the earliest start date first.
5. STR-slack time remaining. This is calculated as the difference between the time
remaining before the due date minus the processing dme remaining. Orders with the shortest STR are run first.
6. STRIOP-Slack time remaining per operation. Orders with shortest STRIP are run first, calculated as follows:
STR/OP = Time remaining before due date - Remaining processing time
Number of remaining operations
7. CR-critical ratio. This is calculated as the difference between the due date and the current date divided by the work remaining. Orders with the smallest CR are run
8. QR-queue ratio. This is calculated as the slack time remaining in the schedule
divided by the planned remaining queue time. Orders with the smallest QR are run
9. LCFS-last-come, first-served. This rule occurs frequently by default. As orders arrive they are placed on the top of the stack and the operator usually picks up the order on top to run first.
10. Random order-whim. The supervisors or th e operators usually select whichever jobthey feel like running.?