When several jobs compete for the capacity of a machine or a work center, the question of sequencing the jobs rises. This question is answered by determining the priority for all the jobs waiting in the queue by applying priority sequencing rules.
The priority indicates the sequences in which the jobs will be processed on the machine or in the work center. When the machine or work center becomes free, the job with the highest priority is assigned.
Choice of the right sequencing rule based on one criterion becomes quite difficult as no single rule is ideal for all situations.
Some of the criteria used are:
1. Set up costs or change over costs.
2. Work-in-progress inventory cost.
3. Idle time.
4. Number or percent of jobs late.
5. Average job lateness.
6. Average flow time.
7. Average number of jobs in the system.
8. Average time to compete a job.
Single-criterion Priority Sequencing Rules:
(a) First come first served (FCFS)
(b) Shortest processing time (SPT) or shortest operation time (SOT) or minimum processing time (MINPRT).
(c) Longest processing time (LPT) or longest operation time (LOT)
(d) Least slack job (LS) first or minimum slack (MIN SLACK) job first.
(e) Earliest due date (EDD) job first.
(f) Truncated shortest processing job first (TSPT).
(g) Preferred customer order (PCO) first.
(h) Random selection (RS)
(i) COVERT ( cost over time)
(j) Least change over cost.
The rules of single criterion sequencing are explained in detail in the following paragraphs:
i) First come-first served (FCFS) rule: Jobs are scheduled for work in the same sequence as they arrive at the facility or work center. This rule is commonly applied in service centers such s banks, super bazaars and barber shops.
ii) Shortest process time job (SPT) first: The job which has the shortest processing/ operation time on the mchine or at the work center is given the highest priority to be loaded as the next job for processing. This rule minimizes the in-process-inventory, however, at the expense of keeping the jobs having longer processing time for a longer time in the work center, there-by increasing the job through-put time (i.e. manufacturing cycle time).
iii) Longest processing time (LPT) job first: The job with the longest processing/ operation time is scheduled as the first job to be loaded on the machine among the jobs waiting in queue.
iv) Least slack (LS) job first: In this rule, the highest priority is given to the job which has the least slack. Slack is the difference between available time and the duration of processing the job.
Slack = Available time â€“ Processing time.
v) Earliest due date (EDD) job first:
This rule sequences the jobs waiting in the queue at the work centre or machine according to their due dates and the jobs are processed according to their due dates i.e. having earliest due date is given highest priority while loading the job on the machine (or work center). This rule does not ensure that all jobs will be completed on time i.e. within their due dates.
vi) Truncated shortest processing time (TSPT) job first: This rule sequences the jobs according to the SPT rule, except that the jobs that have been waiting for time period longer than a specified truncation time are given higher priority than other jobs.
vii) Preferred customer order (PCO) rule: Jobs belonging to a preferred customer are given higher priority than other jobs.
viii) Random-selection: This rule is not used normally. It may be used when no other consideration is important.
ix) COVERT (cost over time) rule: This rule uses the ratio of expected delay cost (C) to the processing time (T). The job with the largest ratio is given the highest priority.
Cost over time ratio = Expected delay cost (C)/Processing time (T)
x) Least change-over cost: The sequencing of jobs is one by analyzing the total cost of making all the machines change over between jobs.