The following table contains information concerning four jobs that are awaiting
ID: 469318 • Letter: T
Question
The following table contains information concerning four jobs that are awaiting processing at a work center. Job Job Time (Days) Due Date (Days) A 14 21 B 10 16 C 7 15 D 5 17 a. Sequence the jobs using (1) First come, first served, (2) Shortest processing time, (3) Earliest due date, and (4) Critical ratio. Assume the list is by order of arrival. Sequence for First come, first served Sequence for Shortest processing time Sequence for Earliest due date Sequence for Critical ratio b. For each of the methods in part a, determine (1) the average job flow time, (2) the average tardiness, and (3) the average number of jobs at the work center. (Round your intermediate calculations and final answers to 2 decimal places.) First Come, First Served Shortest Processing Time Earliest Due Date Critical Ratio Average job flow time Average tardiness Average number of jobs c. Is one method superior to the others? Yes No
Explanation / Answer
a. and b.
(1) First come, first served.
Job Sequence
Job Time
(in Days)
Due Date
(in Days)
Flow Time
(in Days)
Days Tardy = Flow Time - Due Date
(in Days) (0 if negative)
A
14
21
14
0
B
10
16
24
8
C
7
15
31
16
D
5
17
36
19
Total
36
105
43
(i) Average job flow time = 105 / 4 = 26.25 days.
(ii) Average tardiness = 43 / 4 = 10.75 days.
(iii) Makespan = Total completion time of all the jobs = 36 days.
Average number of jobs at the work center = Total flow time / Makespan = 105 / 36 = 2.92.
(2) Shortest processing time.
Job Sequence
Job Time
(in Days)
Due Date
(in Days)
Flow Time
(in Days)
Days Tardy = Flow Time - Due Date
(in Days) (0 if negative)
D
5
17
5
0
C
7
15
12
0
B
10
16
22
6
A
14
21
36
15
Total
36
75
21
(i) Average job flow time = 75 / 4 = 18.75 days.
(ii) Average tardiness = 21 / 4 = 5.25 days.
(iii) Makespan = Total completion time of all the jobs = 36 days.
Average number of jobs at the work center = Total flow time / Makespan = 75 / 36 = 2.08.
(3) Earliest due date.
Job Sequence
Job Time
(in Days)
Due Date
(in Days)
Flow Time
(in Days)
Days Tardy = Flow Time - Due Date
(in Days) (0 if negative)
C
7
15
7
0
B
10
16
17
1
D
5
17
22
5
A
14
21
36
15
Total
36
82
21
(i) Average job flow time = 82 / 4 = 20.5 days.
(ii) Average tardiness = 21 / 4 = 5.25 days.
(iii) Makespan = Total completion time of all the jobs = 36 days.
Average number of jobs at the work center = Total flow time / Makespan = 82 / 36 = 2.28.
(4) Critical ratio.
Job
Job Time
(in Days)
Due Date
(in Days)
Critical ratio = (Remaining time until due date) / (Remaining job time)
Priority
A
14
21
(21 – 0 ) / 14 = 1.5
1
B
10
16
(16 – 0 ) / 10 = 1.6
2
C
7
15
(15 – 0 ) / 7 = 2.14
3
D
5
17
(17 – 0 ) / 5 = 3.4
4
So the job sequence = A – B – C – D.
Job Sequence
Job Time
(in Days)
Due Date
(in Days)
Flow Time
(in Days)
Days Tardy = Flow Time - Due Date
(in Days) (0 if negative)
A
14
21
14
0
B
10
16
24
8
C
7
15
31
16
D
5
17
36
19
Total
36
105
43
(i) Average job flow time = 105 / 4 = 26.25 days.
(ii) Average tardiness = 43 / 4 = 10.75 days.
(iii) Makespan = Total completion time of all the jobs = 36 days.
Average number of jobs at the work center = Total flow time / Makespan = 105 / 36 = 2.92.
Rule
Average Flow Time
(in Days)
Average Tardiness
(in Days)
Average number of jobs at the work center
First come, first served
26.25
10.75
2.92
Shortest processing time
18.75
5.25
2.08
Earliest due date
20.5
5.25
2.28
Critical ratio
26.25
10.75
2.92
c. Shortest processing time method is superior to others as it has the lowest Average Flow Time, Average Tardiness and Average number of jobs at the work center, compared to all the other three methods.
Job Sequence
Job Time
(in Days)
Due Date
(in Days)
Flow Time
(in Days)
Days Tardy = Flow Time - Due Date
(in Days) (0 if negative)
A
14
21
14
0
B
10
16
24
8
C
7
15
31
16
D
5
17
36
19
Total
36
105
43
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