The exponentially weighted moving average (EWMA) rule compared with traditionally used quality control rules
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Kristian Linnet
Abstract
Background: The exponentially moving average (EWMA) rule for internal quality control is a well-known type of control rule in industry. Here, the power of the EWMA rule is evaluated to outline the potential of this type of control rule in clinical chemistry.
Methods: Using simulations, the power of the EWMA rule was explicitly compared with that of commonly used rules in clinical chemistry. The type I error levels were standardized to common values to achieve unbiased comparisons.
Results: For small to moderately large errors (systematic errors up to 2–3 standard deviations), the EWMA rule outperforms simple rules (N=1) and multi-rules (N=2–6). For example, for a systematic error of 2s, the EWMA rule equivalent to the 13s rule has a power of 0.30, whereas the 13s rule only displays a power of approximately 0.15. For N=4, comparison was carried out with the 13s/22s/R4s/41s rule. Here the common type I error level is 0.017. At all error levels, the EWMA rule is superior to the multi-rule. For example, given a 1s systematic error, the EWMA rule has a power (0.4) of twice the value of the multi-rule (0.2).
Conclusion: The EWMA rule is an efficient control rule with regard to systematic errors that should be considered for general application in the field of clinical chemistry.
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©2006 by Walter de Gruyter Berlin New York
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