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These factors for variables control charts (X-bar and R; X-bar and S) can be found in Statistical Quality Control (Montgomery) in Appendix VI.
It is worth noting that many folks believe that these control chart factors are constants. But they are not. They are actually distributions.
R code at Range Estimation of Standard Deviation provides a simulation-based method of deriving the factor table values for "d2", and for demonstrating the statistical nature of d2. The simulation also provides some insight into when we might choose an X-bar and R chart vs. an X-bar and S chart.
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