Using the U.S. Geological Survey National Water Quality Laboratory LT-MDL to Evaluate and Analyze Data

Abstract

A long-term method detection level (LT-MDL) and laboratory reporting level (LRL) are used by the U.S. Geological Survey’s National Water Quality Laboratory (NWQL) when reporting results from most chemical analyses of water samples. Changing to this method provided data users with additional information about their data and often resulted in more reported values in the low concentration range. Before this method was implemented, many of these values would have been censored. The use of the LT-MDL and LRL presents some challenges for the data user. Interpreting data in the low concentration range increases the need for adequate quality assurance because even small contamination or recovery problems can be relatively large compared to concentrations near the LT-MDL and LRL. In addition, the definition of the LT-MDL, as well as the inclusion of low values, can result in complex data sets with multiple censoring levels and reported values that are less than a censoring level. Improper interpretation or statistical manipulation of low-range results in these data sets can result in bias and incorrect conclusions. This document is designed to help data users use and interpret data reported with the LTMDL/LRL method. The calculation and application of the LT-MDL and LRL are described. This document shows how to extract statistical information from the LT-MDL and LRL and how to use that information in USGS investigations, such as assessing the quality of field data, interpreting field data, and planning data collection for new projects. A set of 19 detailed examples are included in this document to help data users think about their data and properly interpret lowrange data without introducing bias. Although this document is not meant to be a comprehensive resource of statistical methods, several useful methods of analyzing censored data are demonstrated, including Regression on Order Statistics and Kaplan-Meier Estimation. These two statistical methods handle complex censored data sets without resorting to substitution, thereby avoiding a common source of bias and inaccuracy. INTRODUCTION In FY 2000, the U.S. Geological Survey’s National Water Quality Laboratory (NWQL) began routinely applying a new reporting procedure for high-demand water methods (Childress and others, 1999). Use of this reporting procedure has continued and is being employed for most analyses on water samples. The reporting procedure does not alter the actual analytical methods used by the NWQL, but only the way in which the results are communicated. Understanding this procedure provides the data user additional information about their data; it also presents the data user with new opportunities and challenges regarding data interpretation, especially in the low range of the method. Moreover, improperly interpreting low range results can bias summary statistics.