Abstract
A typical laboratory control chart provides a means for detecting any intolerable change in the precision or bias of an analytical method away from its historical performance level. But in many laboratories, it would be more desirable to control accuracy, rather than either precision or bias. Thus, a procedure is given here for preparing an accuracy (or more correctly, inaccuracy) control chart for an analytical method. The procedure assumes that the user knows the minimum accuracy that the method is required to exhibit. In this case, knowledge of the prior performance characteristics of the analytical method is not required for establishing the control Limits, as is commonly true for other control-charting schemes. This control-charting procedure involves the daily analysis of spiked control samples. The estimation of total method inaccuracy from the control sample data, the computation of control limits, and the plotting of the control parameter (i.e., inaccuracy) on the control chart. Although the control limits are functions of the method's standard deviation, adequate control can be achieved by merely choosing a reasonable value for standard deviation when computing the control limits. This approach permits the user to display the control limits in the traditional manner, i.e., as unbroken horizontal lines on the control chart over a period of many days or weeks. By Monte Carlo simulation of control-sample data with predefined inaccuracy, bias, and imprecision and by application of the proposed control-charting procedure to the simulated data, it was demonstrated that the procedure exerts satisfactory control over method inaccuracy whether or not an assumed standard deviation is used for computing control limits.