5.4 Prediction Limits

Prediction limits have several uses in groundwater monitoring, all of which involve predicting the upper limit of possible future values based on a backgroundNatural or baseline groundwater quality at a site that can be characterized by upgradient, historical, or sometimes cross-gradient water quality (Unified Guidance). or baseline data set and comparing that limit to compliance point measurements or statistics. An upper prediction limit is constructed from upgradient or historical data and is designed to equal or exceed a specified number of future comparisons. If any of those values exceed the prediction limit, then the analysis suggests that groundwater concentrations have risen above the background levels. Prediction limits explicitly account for the degree of variation in the background population and the size of the sample of measurements used to construct the limit.

Prediction limits can be constructed with either a parametricA statistical test that depends upon or assumes observations from a particular probability distribution or distributions (Unified Guidance). or nonparametricStatistical test that does not depend on knowledge of the distribution of the sampled population (Unified Guidance). statistical model. Parametric prediction limitsIntervals constructed to contain the next few sample values or statistics within a known probability (Unified Guidance). are based on the meanThe arithmetic average of a sample set that estimates the middle of a statistical distribution (Unified Guidance). and standard deviation of the background or baseline data set, whereas nonparametric prediction limits are based on ranking of the observations. Table F-2 includes information about checking assumptions for prediction limits.

Interwell prediction limits compare background and compliance data collected from distinct spatial locations (upgradient versus downgradient). Intrawell prediction limits compare historical data (labeled intrawellComparison of measurements over time at one monitoring well (Unified Guidance). background) versus current data from a single location (see Section 3.6.5: Should I use interwell or intrawell sampling?).

Publication Date: December 2013

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