Exponentially weighted standard deviation

Each result was evaluated on-line, and a limit violation was immediately reported.

The proposed algorithm was implemented in a computer program running on computers to which the analyzers were linked on-line. The probability of reporting a test result with an analytical error that exceeds the total allowable error was calculated for the proposed system as well as for the Westgard rules. This is mainly caused by the introduction of the starting rule of 12s, which proves essential to prevent a large number of rule violations. The Westgard rules perform well, despite the underlying presumption of ergodicity. The proposed system was compared with Westgard rules. The weighting factor was based on a process time constant, which was estimated from the period between two calibration or maintenance procedures. The normalized values were used in the exponentially weighted algorithm. Individual control sample results were normalized to a target value and target standard deviation. The examples given are real, not simulated, data. Limits were defined in accordance with state of the art- or biological variance-derived cut-offs. The process standard deviation was estimated in terms of within-run standard deviation. Process means were estimated overall and for each control level. An exponentially weighted moving average and standard deviation was used to estimate process mean and standard deviation. A model is proposed to assess the status of a process. This behavior is shown in five examples of different analytical systems: alkaline phosphatase on the Hitachi 911 analyzer (Roche), vitamin B12 on the Access analyzer (Beckman), prothrombin time and activated partial thromboplastin time on the STA Compact analyzer (Roche) and PO2 on the ABL 520 analyzer (Radiometer).

The variation can be different at different levels of concentration.

Both the process mean and the process standard deviation vary. The analytical processes in clinical laboratories should be considered to be non-stationary, non-ergodic and probably non-stochastic processes.

Exponentially weighted standard deviation