13. Scheduling

1. Scheduling : pertains to establishing the timing of the use of specific resources (equipment, facilities, and human activities) of an organization.

2. In manufacturing systems, scheduling is about ordering (sequencing) tasks:

-Order tasks to minimize the completion time

3. In service systems, scheduling is about a policy how to serve customers (priority rules):

-FCFS (First Come First Served)

-LCFS (Last Come First Served)

-Shortest Processing Time (SPT)

4. Objective of Scheduling

Minimize processing and setup costs

Minimize idle time at the work centers

Minimize job completion time

5. Measuring the Properties of the System

Job flow time: The length of time a job is in the shop at a particular workstation or work center.  It includes processing time and waiting time.

Average flow time: Average flow time per job.

Job lateness/tardiness: The delay calculated by the difference between the actual completion time and the due date. 0 is assigned if finished before the due date.

Average tardiness: Average tardiness per job.

Makespan: The total time needed to complete a group of jobs.

Average number of jobs: Jobs that are in a shop are considered to be work-in-process inventory.  The average number of jobs is the total flow time divided by the makespan.

6. Priority Rules Used in Business

FCFS - first come, first served

LCFS – last come, first served

SPT - shortest processing time

DD - due date

CR - critical ratio

Rush – emergency

7. Critical Ratio

CR = (Due Date – Start Time)/(Process Time)

The smaller the ratio, the more critical because you have less time to complete the job.

You recalculate the ratio every time you need to decide which job to do next.

CR is smaller if Due Date is closer to the Start Time and/or Process Time ( = amount of work to do!) is large.