This document is also available in pdf format:
Open-File Report 99-193.pdf. (Note an erratum for page 15.)

New Reporting Procedures Based on Long-Term Method Detection Levels and Some Considerations for Interpretations of Water-Quality Data Provided by the U.S. Geological Survey National Water Quality Laboratory

By Carolyn J. Oblinger Childress, William T. Foreman, Brooke F. Connor, and Thomas J. Maloney

Table of Contents

Erratum for Open-File Report 99-193

Cover2 (gif file 6K)
Front page (gif file 4K)
Front2 Page (gif file 6K)

The minimum reporting level
     Historical use of the minimum reporting level at the National Water Quality Laboratory
     Limitations of the minimum reporting level
The method detection limit procedure of the U.S. Environmental Protection Agency
     Limitations of the procedure
     Use by the National Water Quality Laboratory
The U.S. Geological Survey’s new reporting procedure based on long-term method detection levels
Setting the reporting level
New reporting conventions
    Information-rich methods
     Use of the “E” remark code by the National Water Quality Laboratory
Planned implementation at the National Water Quality Laboratory
Considerations for data interpretation
     Effect of censoring on data distribution
     Data analysis
Storing data in the National Water Information System
     Enhancements to remark code in the National Water Information System
     Impact on historical data
Explanation of the reporting convention for long-term method detection levels for the State annual data reports
Abbreviations and definitions


  1. Diagram showing the spike concentration in relation to the expected method detection limit (MDL)
  2. Graph showing the frequency distribution of measured concentrations of chlorobenzene spiked at 0.05 microgram per liter
  3. Frequency distibution of measured concentrations of method detection limit (MDL) test samples spiked at 1 to 5 times the expected MDL concentration and showing one standard deviation (s)
  4. Standard deviation in relation to concentration of analyte showing a region of constant standard deviation at low concentrations
  5. The frequency distribution of the low-concentration spike measurements is centered on zero concentration to simulate the distribution expected for replicate blank measurements (analyte not present).
  6. The U.S. Environmental Protection Agency’s method detection limit (MDL) is set at a concentration to provide a false positive rate of no more than 1 percent.
  7. The long-term method detection level (LT–MDL) compared to the method detection limit (MDL) determined by the U.S. Environmental Protection Agency procedure
  8. False negative probability when a sample contains the analyte at the method detection limit (MDL) concentration
  9. The risk of a false negative (not detecting an analyte when it is present) at the laboratory reporting level (LRL) is no more than 1 percent.
  10. New low-concentration reporting conventions showing the reported value and associated qualifying remark code in relation to the long-term method detection level (LT–MDL), the laboratory reporting level (LRL), and the lowest calibration standard (LS).
  11. One-dimensional distribution plots of (A) uncensored data and the same data set censored by using (B) the MRL procedure, (C) the new reporting procedure, and (D) the new reporting procedure for information-rich methods.


  1. Examples of different uses for data and their effect on the required level of certainty and use of qualified data

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