Control charts are used to calculate control limits and estimate the process standard deviation. Control chart constants used are D4, D3, A2, and d2 (These control chart constants depend on the subgroup size n).
These control chart constants are summarised in the table below. For example, if your subgroup is 4, then D4 = 2.282, A2 = 0.729, and d2 = 2.059.
There is no value for D3. This simply means that the R chart has no lower control limit when the subgroup size is 4.
Sub group Size (n) | A2 | D3 | D4 | d2 |
---|---|---|---|---|
2 | 1.880 | 3.267 | 1.128 | |
3 | 1.023 | 2.574 | 1.693 | |
4 | 0.729 | 2.282 | 2.059 | |
5 | 0.577 | 2.114 | 2.326 | |
6 | 0.483 | 2.004 | 2.534 | |
7 | 0.419 | 0.076 | 1.924 | 2.704 |
8 | 0.373 | 0.136 | 1.864 | 2.847 |
9 | 0.337 | 0.184 | 1.816 | 2.970 |
10 | 0.308 | 0.223 | 1.777 | 3.078 |
11 | 0.285 | 0.256 | 1.774 | 3.173 |
12 | 0.266 | 0.284 | 1.716 | 3.258 |
13 | 0.249 | 0.308 | 1.692 | 3.336 |
14 | 0.235 | 0.329 | 1.671 | 3.407 |
15 | 0.223 | 0.348 | 1.652 | 3.472 |
16 | 0.212 | 0.364 | 1.636 | 3.532 |
17 | 0.203 | 0.379 | 1.621 | 3.588 |
18 | 0.194 | 0.392 | 1.608 | 3.640 |
19 | 0.187 | 0.404 | 1.596 | 3.689 |
20 | 0.180 | 0.414 | 1.586 | 3.735 |
21 | 0.173 | 0.425 | 1.575 | 3.778 |
22 | 0.167 | 0.434 | 1.566 | 3.819 |
23 | 0.162 | 0.443 | 1.557 | 3.858 |
24 | 0.157 | 0.452 | 1.548 | 3.895 |
25 | 0.153 | 0.459 | 1.541 | 3.931 |
Thanks
Values greater than 25?
Will update the article soon