Levey-Jennings charts are often used in laboratories to ensure that test methods are accurate. A control, with a known “true value,” is selected for a lab test, and is run on a regular basis (e.g., each shift, daily, or before each lab test). Each control result is plotted on the chart, and the generated standard deviation is utilized to set the control limits. Special rules, called the “Westgard Rules,” are used to interpret the Levey-Jennings chart.