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 xâ¯â¯ and R charts

FACTORS FOR R CHARTS
Number of Observations n Factor for Chart, LCL UCL
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

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