Question

A machine shop has received four jobs A, B, C and D for processing on a single CNC machine. All jobs are available for processing on the first day of the production schedule calendar, and processing times and due dates as applicable on the first day are given below. Using earliest due date rule, the average tardiness (in days) is _______. [in integer]

Job	Processing time (in days)	Due date (day)
A	8	14
B	5	10
C	7	12
D	9	19

Answer 1

Concept:

Earliest Due Date (EDD): The sequence of the job is based on ascending order of the due date.

• Processing time: The time spent by a job on a machine.
• Due date: Expected completion time of a job.
• Flow time: Time spends by a job on the shop floor. Cumulative addition of Processing time.
• Make span: Time interval between starting of 1st job & coming out of the last job in all machine.
• Earliness/Tardiness: Time difference between Flow time and Due date time.
  • Earliness: When the time difference is negative i.e. Job comes out before the due date
    • Put ‘0’ under tardiness.
  • Tardiness: When the time difference is positive i.e. Job comes out after the due date.
    • Put ‘0’ under earliness.

Calculation:

Arranging the jobs in ascending order of due date,

Job	Processing Time	Flow Time	Due date	Tardiness
B	5	0 + 5 = 5	10	0
C	7	5 + 7 = 12	12	0
A	8	12 + 8 =  20	14	6
D	9	20 + 9 = 29	19	10

∴ Total tardiness = 6 + 10 = 16

∴ Average Tardiness = \(\rm {Total \;Tardiness} \over No. \;of\; jobs\)

= \(16\over 4\) = 4

Method	Definition
First Come First Serve (FCFS)	Jobs are arranged as per they arrived
Shortest Processing Time (SPT)	Jobs are arranged as per the ascending order of Processing Time.
Earliest Due Date (EDD)	Jobs are arranged as per the ascending order of Due date.
Critical Ratio (CR)	\(\rm C.R = \frac{{Due\;date}}{{Procesisng\;Time}}\) Jobs are arranged as per the ascending order of Critical Ratio.