Sample sigma vs. process sigma

07/03/2008:

Is it possible to use sample sigma instead of process sigma when calculating control limits?
Ed S.

You do not want to use sample sigma when calculating control limits, as this would not then be a control chart. Statistical control charts provide the ability to detect process trends by using the short term variation as a predictor for the longer term variation. The short-term variation is measured within the subgroup (or between successive subgroups in the case of subgroup size of one), and identified by the nomenclature process sigma. Intrinsic to this definition of statistical process control is the time-ordered nature of the process data. The sample sigma calculation ignores the time-ordering of the data, preventing it from being useful for statistical process control estimates.

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