Measurements of trace chemicals in environmental media (water, air, soils, biota) frequently result in values reported only as less than the laboratory reporting limit (“nondetects”, or “qualified values”). The most commonly-used method for incorporating nondetects is to substitute one-half the reporting limit and continue as usual. This produces invasive data that may obscure patterns and trends that are present, or create those that are not present in the original data. It is fraught with error.
Standard methods for interpreting this type of data exist in medical and industrial applications, but have rarely been applied to environmental data. Methods are available for computing summary statistics, hypothesis tests, and regression equations. Their results are unequivocal, powerful, and accurate. This course introduces methods from the author’s textbook Nondetects And Data Analysis: Statistics for censored environmental data, published in 2005 by John Wiley.
Topics for the session:
- Why not use 1/2 the detection limit as a substitute for nondetect values. What are the problems? Are they serious? (yes, they are)
- What better methods are out there, and how do they work?
- What resources are available for the practical scientist (papers, software, etc.)?
- How best to compute the mean and its UCL95 (as an example of a common analysis task) for data with nondetects?
Who should attend?
The most important requirement is an interest in and need for interpreting nondetect data correctly. Though some familiarity with basic statistics will be helpful, there are no prerequisites.
Instructor:
Dr. Dennis Helsel (PhD, Environmental Science and Engineering, Virginia Tech) has 30 years experience applying statistics to environmental science. He is the lead author of the popular textbook "Statistical Methods in Water Resources" (USGS,2002) and of "Nondetects And Data Analysis" (Wiley, 2005) as well as many technical articles. Dr. Helsel was the 2003 recipient of the Distinguished Achievement Award from the American Statistical Association's section on Statistics and the Environment, and of the Dept. of Interior's Distinguished Service Award in 2007. He has trained scientists in the US and internationally since 1990, and does statistical consulting through his company, Practical Stats.
Hotel Information:
DoubleTree Ontario Hotel
222 North Vineyard Avenue
Ontario, CA 91764
Tel: (909) 937-0900
The GRA group guest room rate is $89 plus tax, and is available until October 21, 2010. Mention the GRA Group Rate when reserving your room.
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