5.13 Control Charts

Control charts may be used as an alternative to parametricA statistical test that depends upon or assumes observations from a particular probability distribution or distributions (Unified Guidance). prediction limits for detection monitoring purposes and are commonly used to monitor the stability of groundwater data and to detect changes in data trends that may require further investigation. Control charts offer an advantage over prediction limitsIntervals constructed to contain the next few sample values or statistics within a known probability (Unified Guidance). because they generate a graph of compliance data over time and allow for better identification of long-term trends. To generate control chartsGraphical plots of compliance measurements over time; alternative to prediction limits (Unified Guidance)., a control limit is estimated from backgroundNatural or baseline groundwater quality at a site that can be characterized by upgradient, historical, or sometimes cross-gradient water quality (Unified Guidance). data and subsequently compared to a set of compliance point measurements. A calculated comparison value that exceeds the control limit suggests that compliance point concentrations exceed background. Control charts may be constructed as interwellComparisons between two monitoring wells separated spatially (Unified Guidance). or intrawellComparison of measurements over time at one monitoring well (Unified Guidance). comparisons; background data are collected from upgradient or other background wells for interwell comparisons, and from historical measurements at a targeted compliance well for intrawell comparisons. Baseline parameters (estimates of the meanThe arithmetic average of a sample set that estimates the middle of a statistical distribution (Unified Guidance). and standard deviation) are obtained from the background data. As future compliance observations are collected, the baseline parameters are used to standardize the newly gathered data. A new observation is considered “out of control” if it exceeds the baseline control limits, thus indicating a spike or significant change in the trend of the data.

Examples of control chart tests include the Shewart control limit and the cumulative sum control chart (CUSUM). The Shewart control limit tests for and flags a sudden spike or change in trend of the data, which may indicate an event such as a new release at the site. The CUSUM control limit tests for and flags a gradual, but significant, increase or decrease over time, which may, for example, indicate plume migration.

Publication Date: December 2013

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