The time to replace vehicle wiper blades at a service center was monitored using a mean and a range chart. Six samples of n = 20 observations have been obtained and the sample means and ranges computed:

Sample	Mean	Range	Sample	Mean	Range
1	3.06	.42	4	3.13	.46
2	3.15	.50	5	3.06	.46
3	3.11	.41	6	3.09	.45

Factors for three-sigma control limits for and R charts

		FACTORS FOR R CHARTS
Number of Observations in Subgroup, n	Factor for Chart, A2	Lower Control Limit, D3	Upper Control Limit, D4
2	1.88	0	3.27
3	1.02	0	2.57
4	0.73	0	2.28
5	0.58	0	2.11
6	0.48	0	2.00
7	0.42	0.08	1.92
8	0.37	0.14	1.86
9	0.34	0.18	1.82
10	0.31	0.22	1.78
11	0.29	0.26	1.74
12	0.27	0.28	1.72
13	0.25	0.31	1.69
14	0.24	0.33	1.67
15	0.22	0.35	1.65
16	0.21	0.36	1.64
17	0.20	0.38	1.62
18	0.19	0.39	1.61
19	0.19	0.40	1.60
20	0.18	0.41	1.59

a. Using the factors in the above table, determine upper and lower limits for mean and range charts.

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a Calculate the value of average mean X and average ... View the full answer