Jack's Refrigeration Repair is under contract to repair, recondition, and/or refurbish commercial and industrial icemakers from restaurants, seafood processors, and similar organizations

Jack currently has five jobs to be scheduled, shown in the order in which they arrived.

Job Processing Time (hours) Due (hours)
V 20 50
W 10 35
X 50 90
Y 15 35
Z 55 75

(a) Complete the following table. (Show your supporting calculations below).
(b) Which dispatching rule has the best score for flow time?
(c) Which dispatching rule has the best score for work-in-process (jobs in the system)?
(d) Which dispatching rule has the best score for lateness?

Dispatching Rule Job Sequence Average Flow Time Average Number of Jobs Average Lateness
FCFS
SPT
EDD
CR

(a) A summary of calculations appears in the table below.

Dispatching Rule Job Sequence Average Flow Time Average Number of Jobs Average Lateness
FCFS V, W, X, Y, Z 75 2.50 27
SPT W, Y, V, X, Z 65 2.17 16
EDD W, Y, V, Z, X 66 2.20 17
CR Z, X, Y, V, W 114 3.80 61

(b, c, d) SPT is best on all three criteria: flow time, work-in-process, and lateness. Supporting calculations for each priority rule appear in the tables below.

SPT Machine1 Due Date Flow Time Late
W 10 35 10 0
Y 15 35 25 0
V 20 50 45 0
X 50 90 95 5
Z 55 75 150 75
Total 325 80
Average 65 16
Average # jobs in system = 2.17

FCFS Machine1 Due Date Flow Time Late
V 20 50 20 0
W 10 35 30 0
X 50 90 80 0
Y 15 35 95 60
Z 55 75 150 75
Total 375 135
Average 75 27
Average # jobs in system = 2.5

EDD Machine1 Due Date Flow Time Late
W 10 35 10 0
Y 15 35 25 0
V 20 50 45 0
Z 55 75 100 25
X 50 90 150 60
Total 330 85
Average 66 17
Average # jobs in system = 2.20

CR Machine1 Due Date CR Flow Time Late
Z 55 75 1.36 55 0
X 50 90 1.8 105 15
Y 15 35 2.33 120 85
V 20 50 2.5 140 90
W 10 35 3.5 150 115
Total 570 305
Average 114 6

Average # jobs in system = 3.80