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Interpreting Non-Detect Data Correctly
Hosted by U.S. EPA

For most environmental professionals, the way to deal with "nondetects" is less than obvious. Values below detection or reporting limits result from measuring trace amounts of a variety of organic and inorganic chemicals. This course provides an overview of up-to-date (maximum likelihood and survival analysis) methods for computing summary statistics, hypothesis tests, and regression for data with one or more detection limits. The focus will be on what not do, the problems with substituting numbers, and what software is available. The course assumes a knowledge of basic statistics, including some familiarity with t-tests, linear regression, and simple nonparametric tests like the rank-sum test. Participants will receive a CD with further information, such as journal articles, available free software, and other references.

INSTRUCTOR: Dennis Helsel (PhD in Environmental Science and Engineering, Virginia Tech) has 29+ years experience in applying statistical methods to environmental sciences. He has authored articles in numerous journals, including papers on handling nondetect data such as "Less Than Obvious" (ES&T, 1990) and "More Than Obvious" (ES&T 2005). Dr. Helsel has authored Nondetects and Data Analysis (Wiley, 2005), the textbook for this course, as well as Statistical Methods in Water Resources (USGS, 2002). Dr. Helsel is a 2003 recipient of the Distinguished Service Award from the American Statistical Association's section on Statistics and the Environment.
For general information contact Jerri Town (EMS, Inc.) by telephone at 301-589-5318 , Ext. 25 or via e-mail at

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