Consider the forecast errors in Exercise 2.34. Construct a cumulative sum control chart for these forecast errors. Does the forecasting system exhibit stability over this time period?
Exercise 2.34Table E2.1 contains 40 one-step-ahead forecast errors from a forecasting model.
TABLE E2.1 Chemical Process Viscosity Readings
Time
Period
Reading
1
86.7418
26
87.2397
51
85.5722
76
84.7052
2
85.3195
27
87.5219
52
83.7935
77
83.8168
3
84.7355
28
86.4992
53
84.3706
n
82.4171
4
85.1113
29
85.6050
54
83.3762
79
83.0420
5
85.1487
30
86.8293
55
84.9975
80
83.6993
6
84.4775
31
84.5004
56
84.3495
81
82.2033
7
84.6827
32
84.1844
57
85.3395
82
82.1413
8
84.6757
33
85.4563
58
86.0503
83
81.7961
9
86.3169
34
86.1511
59
84.8839
84
82.3241
10
88.0006
35
86.4142
60
85.4176
85
81.5316
11
86.2597
36
86.0498
61
84.2309
86
81.7280
12
85.8286
37
86.6642
62
83.5761
87
82.5375
13
83.7500
38
84.7289
63
84.1343
88
82.3877
14
84.4628
39
85.9523
64
82.6974
89
82.4159
15
84.6476
40
86.8473
65
83.5454
90
82.2102
16
84.5751
41
88.4250
66
86.4714
91
82.7673
17
82.2473
42
89.6481
67
86.2143
92
83.1234
18
83.3774
43
87.8566
68
87.0215
93
83.2203
19
83.5385
44
88.4997
69
86.6504
94
84.4510
20
85.1620
45
87.0622
70
85.7082
95
84.9145
21
83.7881
46
85.1973
71
86.1504
96
85.7609
22
84.0421
47
85.0767
72
85.8032
97
85.2302
23
84.1023
48
84.4362
73
85.6197
98
86.7312
24
84.8495
49
84.2112
74
84.2339
99
87.0048
25
87.6416
50
85.9952
75
83.5737
100
85.0572
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