Standard Deviation differences between IR and EWMA control charts

Valerie_Nedbal

Hello all,

Does somebody know why is there a difference of Standard Deviation values between IR and EWMA control charts limits (go to control chart > save limits in a new table). Is that somewhere documented?

Thanks in advance,

Valerie

Mark_Bailey

These charts are different and so they use different estimators for the process sigma. The computations are documented for both of these charts: Help > Books > Quality and Process Methods. See chapter 3: Control Chart Builder for statistical details of IR chart starting on page 66. See chapter 4 for details about EWMA starting on page 108.