This question comes from Robbie Keith of Summit Laboratory We are in the process of evaluating our QC program. Our techs monitor Levy-Jennings charts for shifts and trends weekly. We would like to know what you consider to define a shift or trend (e.g. how many points are required increasing or decreasing to define a trend?) Consider control rules such as 41s, 10mean, etc., as good indicators of shifts and trends. The number of observations needed increases as the limit approaches the mean of the control material in order to keep the false rejections down. Minimum number of consecutive observations above or below the mean should probably be set as 6. There are some recommendations, particularly in the Germany, to use 7 above or below the mean, or 7 trending consecutively in one direction.

Links to India, Part I: Best QC Practices and "Westgard Rules"

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