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METHODS OF DEALING WITH VALUES BELOW THE LIMIT OF DETECTION USING SAS
Citation:
Croghan, C AND P P. Egeghy. METHODS OF DEALING WITH VALUES BELOW THE LIMIT OF DETECTION USING SAS. Presented at Southern SAS User Group, St. Petersburg, FL, September 22-24, 2003.
Impact/Purpose:
- Developing publically accessible databases.
- isseminating human exposure data bases and data tools in an appropriate manner to improve human exposure research and information.
- Ensuring that human exposure data sets generated by HEASD and its contractors or collaborators are useful for data analysis and for human exposure modeling, by providing formats and guidance on data base creation, documentation, storage, and retrieval.
- Ensuring that data sets produced in HEASD conform with ORD and EPA requirements.
- Compiling and translating existing human exposure data sets into useful formats for use by HEASD scientists and modelers.
- Making fully documented human exposure data sets available for researchers with collaborating external organizations.
Description:
Due to limitations of chemical analysis procedures, small values cannot be precisely measured. These values are said to be below the limit of detection (LOD). In statistical analyses, these values are often censored and substituted with a constant value, such as half the LOD, the LOD divided by the square root of 2, or zero. This method for handling below-detection values produces a distribution that is the concatenation of two distributions, a uniform distribution for those values below the LOD and the true distribution. Depending upon the percentage of values below the LOD this can produce questionable descriptive statistics. An alternative method uses the distribution of the values above the LOD to estimate the values below the LOD. An example program using the same data is presented calculating the geometric mean, geometric standard deviation, and t-test for both methods to compare the results.
This work has been funded wholly by the United States Environmental Protection Agency. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products do not constitute endorsement or recommendation for use.