The Levey-Jennings chart is a form of statistical process control (SPC) graph commonly employed in laboratory quality control contexts. Unlike the Shewhart individuals control chart, it relies on the long-term estimate of sigma instead of the short-term estimate within subgroups or samples. Control limits for proportion data are set at 10-20-30 percent. Instead of using the Nelson Rules, the Westgard Rules are advised to determine whether to release the results obtained from samples taken during the control or to rerun them.