False negative (also called type II error or beta error)— A statement that a substance is not present (was not found) in a sample when the substance was present (Keith, 1992).

False positive (also called type I error or alpha error)— A statement that a substance is present in a sample when it is not (Keith, 1992).

Information-rich methods— Classified as organic methods that use either mass spectrometric or photodiode array ultraviolet/visible spectroscopic detection. These methods have additional qualifying information that allows enhanced analyte identification.

Laboratory reporting level (LRL)— Generally equal to twice the yearly determined LT–MDL. The LRL controls false negative error. The probability of falsely reporting a non-detection for a sample that contained an analyte at a concentration equal to or greater than the LRL is predicted to be less than or equal to 1 percent. The value of the LRL will be reported with a “ less than ” remark code for samples in which the analyte was not detected. The National Water Quality Laboratory collects quality-control data from selected analytical methods on a continuing basis to determine long-term method detection levels (LT–MDL’s) and establish laboratory reporting levels (LRL’s). These values are re-evaluated annually based on the most current quality-control data and may, therefore, change. [Note: In several previous NWQL documents (Connor and others, 1998; NWQL Technical Memorandum 98.07, 1998), the LRL was called the non-detection value or NDV—a term that is no longer used.]

Long-term method detection level (LT–MDL)— A detection level derived by determining the standard deviation of a minimum of 24 MDL spike sample measurements over an extended period of time. LT–MDL data are collected on a continuous basis to assess year-to-year variations in the LT–MDL. The LT–MDL controls false positive error. The chance of falsely reporting a concentration at or greater than the LT–MDL for a sample that did not contain the analyte is predicted to be less than or equal to 1 percent.

Method detection limit (MDL)— Minimum concentration of a substance that can be measured and reported with 99-percent confidence that the analyte concentration is greater than zero. It is determined from the analysis of a sample in a given matrix containing the analyte (U.S. Environmental Protection Agency, 1997). At the MDL concentration, the risk of a false positive is predicted to be less than or equal to 1 percent.

Minimum reporting level (MRL)— Smallest measured concentration of a constituent that may be reliably reported by using a given analytical method (Timme, 1995).

Non-quantitative result— Unable to report a concentration. Either not detected or detected in a region of high uncertainty (high probability of false positive) and outside the calibration range.

Quantitative result— Concentration reported. Value is within range of instrument calibration and, thus, of higher certainty.

Semi-quantitative result— Estimated concentration reported because it is outside the calibration range.

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up arrow U.S. Geological Survey Open-File Report 99-193