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Title: Operator variance in the determination of the plastic limit
Authors: Washington University (Saint Louis, Mo.). Dept. of Earth Sciences.
Ballard, Geoffrey E. H. (Geoffrey Edwin Hall), 1932-2008
Weeks, W. F.
Keywords: Soils
Cohesive soils
Mechanical properties
Plastic properties
Mathematical analysis
Mathematical models
Publisher: Cold Regions Research and Engineering Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Research report (Cold Regions Research and Engineering Laboratory (U.S.)) ; 117.
Description: Research Report
Summary: An experiment was designed to investigate operator-variance in the determination of the plastic limit of cohesive soils, independent of sample preparation and hydration time. A standard sample was prepared by dry-mixing commercial clay, "Grundite", with a pure silt. After the sample was hydrated for 3 weeks, five random samples were taken and five replicates performed on each sample. Analysis of variance (AOV) shows that there is no reason to doubt that the sample is homogeneous at the 0.05 level of significance. Random samples were then distributed to five zones of operators, where a zone is defined as a group of operators with similar backgrounds and engineering interests. Two operators from each group performed five replicates. The AOV model used in the analysis is mixed with the upper level fixed and the two lower levels random. The estimated "within" and "between" operator variances are 0. 45 and 4. 18 respectively. The estimated "between zones" contribution to the total sum of squares is negative. No correlation was found between an operator's internal variance and his deviation from the grand mean. On the basis of this experiment, no reason-exists to doubt that an "untrained" operator can obtain results comparable to those of professional operators. The major factor contributing to the total sample variance is the inconsistency between the individual operators who, although able to duplicate their own determinations, do not call the same end point. To minimize between operator variance, a readily prepared standard sample is suggested with which any operator can calibrate his plastic limit determinations against the expected national average.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Research Report

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