home > sqc > formulas
Untitled Document
X-bar and sigma chart formulas
X-bar control limits are based on either range or sigma, depending on which chart it is paired with. When the X-bar chart is paired with a sigma chart, the most common (and recommended) method of computing control limits based on 3
standard deviations is:
X-bar
Upper control limit:
|
Lower control limit:
|
Sigma
|
|
Upper control limit:
|
Lower control limit:
|
Subgroup Size |
A3 |
B3 |
B4 |
2 |
2.659 |
----- |
3.267 |
3 |
1.954 |
----- |
2.568 |
4 |
1.628 |
----- |
2.266 |
5 |
1.427 |
----- |
2.089 |
6 |
1.287 |
0.030 |
1.970 |
7 |
1.182 |
0.118 |
1.882 |
8 |
1.099 |
0.185 |
1.815 |
9 |
1.032 |
0.239 |
1.761 |
10 |
0.975 |
0.284 |
1.716 |
11 |
0.927 |
0.321 |
1.679 |
12 |
0.886 |
0.354 |
1.646 |
13 |
0.850 |
0.382 |
1.618 |
14 |
0.817 |
0.406 |
1.594 |
15 |
0.789 |
0.428 |
1.572 |
16 |
0.763 |
0.448 |
1.552 |
17 |
0.739 |
0.466 |
1.534 |
18 |
0.718 |
0.482 |
1.518 |
19 |
0.698 |
0.497 |
1.503 |
20 |
0.680 |
0.510 |
1.490 |
21 |
0.663 |
0.523 |
1.477 |
22 |
0.647 |
0.534 |
1.466 |
23 |
0.638 |
0.545 |
1.455 |
24 |
0.619 |
0.555 |
1.445 |
25 |
0.606 |
0.565 |
1.435 |
|
|
