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 1(3s) rule has a power of 0.30, whereas the 1(3s) 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).


CONCLUSIONS
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.
Linnet
Kristian
K
Department of Forensic Chemistry, University of Copenhagen, Copenhagen, Denmark. linnet@post7.tele.dk
D002624:Chemistry, Clinical / Q000379:methods / Q000592:standards; D015233:Models, Statistical; D011786:Quality Control