The exponentially weighted moving average (EWMA) rule compared with traditionally used quality control rules

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 1(3s)/2(2s)/R-4s/4(1s) 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.