Two commonly used measures of variability which adjust for the magnitude of analyte concentration are coefficient of variation and relative percent difference. Sample standard deviation and coefficient of variation are used when there are at least three replicate measurements. Precision and bias are monitored by plotting control charts to determine if the measurement system is in control. A standard reference material or control sample is selected and analyzed with each batch of unknowns or, if a large number of unknowns is run in a batch, one control sample for each 10 or 20 unknowns. After 15 to 20 analyses, the mean and standard deviation of the data are calculated and a control chart constructed.
Control charts are very useful in visualizing trends.
The chart below is an example of a chart showing drift. The percent recovery is calculated and plotted on the control chart. The control chart lines on the spiked-sample chart correspond to the mean recovery and the 95 and 99 percent limits calculated from the standard deviation of the recovery data. The resultant chart can be used and interpreted in the same way as the X-chart. Since this type of chart would seldom be used for the type of analyses discussed in this manual, it will not be discussed further.