X-Charts

An X-chart has a central line, upper and lower warning limits and upper and lower action limits.

One of the oldest and simplest types of control chart is the X-chart, which is based on the distribution of the control values around a true or expected value. It can be used to monitor the combination of systematic and random effects for control values, based on single results or on a mean of multiple analyses. Using a reference material as control sample, the bias may be monitored by comparing the mean control value over time with the reference value.

X-charts are used to monitor if a bias arise in the results of analysis of a control materials, thus serving to prove whether the method performance remains under stable operation. It is worth to notice that the latter is valid for any sample matrix and elemental concentration order similar to those of the control sample. If a run comprises different types of matrices or different concentration ranges, it is advisable to include more than a control sample on each run.

See an example of X-Shewart Control Chart.