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, |
Factor for |
Lower |
Upper |
|
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.
