Statistics for Censored Environmental Data
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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 presents 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. Example problems are worked in class, so students can confidently take these methods back to their office. 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.
Textbook: Statistics for Censored Environmental Data Using Minitab and R
The second edition of Dennis Helsel's textbook for statistics with censored data (published by Wiley, 2012) is available from your favorite bookseller. New in this edition is a chapter on multivariate methods for censored data, how to sum components of data with nondetects to obtain a total, and extensive use of methods that can incorporate values between detection and reporting limits (censored data that are not 'nondetects'). Code for R as well as Minitab is used throughout the text.
Minitab macros for censored data
Version 4 computes methods from Statistics for Censored Environmental Data using Minitab and R. Several macros reflect the advances in the second edition. Available for free, as always, from our NADA downloads page.
R scripts for censored data
The NADA package for the free R statistical software system is available from the CRAN site.
Dennis Helsel (PhD in Environmental Science and Engineering, Virginia Tech) has over 32 years experience in applying statistical methods to environmental sciences. He has authored articles in numerous journals, including papers on handling nondetect data such as “More Than Obvious” (ES&T 2005) and "Much Ado About Next to Nothing" (Ann. Occ. Hygiene, 2010). Dr. Helsel has authored Statistics for Censored Environmental Data using Minitab and R (Wiley, 2012), 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.
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