If the following jobs are sequenced according to the SPT rule then the mean tardiness (in days)for all jobs is (assume zero for today's date)
A)16 days.
B)13 days.
C)5.2 days.
D)3.2 days

Question

If the following jobs are sequenced according to the SPT rule then the mean tardiness (in days)for all jobs is (assume zero for today's date) 
A)16 days. 
B)13 days. 
C)5.2 days. 
D)3.2 days

Quizplus · Accepted Answer

SPT (Shortest Processing Time) scheduling rule minimizes the mean tardiness. By arranging the jobs in descending order of their processing times (SPT), the sequence will be: Job 4, Job 1, Job 2, Job 3.
Job 4 will complete processing in 2 days, Job 1 in 3 days, Job 2 in 4 days and Job 3 in 6 days. The due dates are: Job 1: 2; Job 2: 3; Job 3: 5; Job 4: 8. 
The job tardiness (number of days late) can be calculated as the difference between the completion date and the due date. 
Job 4 tardiness = 2 days (completed on day 10, due on day 8) 
Job 1 tardiness = 1 day (completed on day 5, due on day 4) 
Job 2 tardiness = 1 day (completed on day 9, due on day 8) 
Job 3 tardiness = 0 days (completed on day 15, due on day 15)
Thus, the mean tardiness = (2 + 1 + 1 + 0)/4 = 1.0 days.
Since the due dates are all before today, add the mean tardiness to today's date.
Therefore, mean tardiness = 1.0 + 4.2 = 5.2 days.

If the Following Jobs Are Sequenced According to the SPT