Control Chart Table of Constants (A2 d2 D3 D4) – A Complete Guide

Control Charts

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).

What are Control Chart Tables of Constants?

Control Chart Tables of Constants are values used for statistical process control. They provide reference points to determine if a process is in-control or out-of-control due to special causes. When control limits have been established, the constants A2, d2, D3 and D4 help to determine these limits and decide whether a process is stable or needs intervention to improve the quality of the product.

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.

Knowing how to use Control Chart Tables of Constants – A2, d2, D3, and D4 – can greatly improve the quality control process. These tables provide useful reference points to determine whether a process is in-control or out-of-control. This guide will detail what each constant means and explore when and how it should be used.

Sub group Size (n)A2D3D4d2
21.880 3.2671.128
31.023 2.5741.693
40.729 2.2822.059
50.577 2.1142.326
60.483 2.0042.534
70.4190.0761.9242.704
80.3730.1361.8642.847
90.3370.1841.8162.970
100.3080.2231.7773.078
110.2850.2561.7743.173
120.2660.2841.7163.258
130.2490.3081.6923.336
140.2350.3291.6713.407
150.2230.3481.6523.472
160.2120.3641.6363.532
170.2030.3791.6213.588
180.1940.3921.6083.640
190.1870.4041.5963.689
200.1800.4141.5863.735
210.1730.4251.5753.778
220.1670.4341.5663.819
230.1620.4431.5573.858
240.1570.4521.5483.895
250.1530.4591.5413.931

3 responses to “Control Chart Table of Constants (A2 d2 D3 D4) – A Complete Guide”

  1. Kanika Yadav

    Thanks

  2. Bao

    Values greater than 25?

    1. Gaurav Yadav

      Will update the article soon

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