Should I use a running mean and SD for establishing control limits instead of basing these limits on the monthly (fixed interval) mean and SD?

The mean and SD calculated from monthly control data do vary (the SD more so than the mean value). A better estimation and representation of the mean and SD for a control lot number is obtained by using more data points collected over a longer period of time. For stable methods, the cumulative value approach, which includes control measurements for a longer, specified period of time eg, 2, 3, 6 months, can be used. Many instruments and/or QC programs automatically perform these calculations. For manual calculations, make sure to use the correct calculations. (See Chapter 13, Calculations, Basic QC Practices, by J.O. Westgard)

Resources: http://www.westgard.com/books-and-reference-manuals/basic-qc-practices-3rd-edition.htm

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