TAPPI T 1205 2014 Edition, January 1, 2014
Dealing with suspect (outlying) test determinations
This TAPPI Standard Practice provides a procedure for judging whether suspect test determinations should be investigated further for possible rejection. A suspect determination (apparent outlier) is one that appears to deviate markedly from other determinations on the same sample of material. An outlying determination (outlier) is a suspect determination for which the deviation has, in fact, been found to be significant using an appropriate statistical test.
Formal treatment of suspect test determinations, as specified in this document, is necessary only in critical situations (e.g., very critical research) or when required by a product specification or an official test method.
Formal treatment of suspect test determinations and test results is highly desirable in studies establishing the repeatability and reproducibility of a test method (see TAPPI T 1200 "Interlaboratory Evaluation of Test Methods to Determine TAPPI Repeatability and Reproducibility").
Both nonstatistical and statistical rules for dealing with suspect test determinations are given. Basically no test determination should be accepted, no matter how correct the value appears to be, if it is known that a faulty determination has been made, and no test determination should be completely rejected purely on a statistical significance test.
The statistical tests described in this practice have been selected from a large number that are available. They apply to the simplest kind of experimental data, that is, replicate determinations of some property of a given sample of material.
NOTE 1: This practice applies to replicate test determinations, usually on several specimens taken under the same conditions and measured in a brief period of time. A test result, obtained in accordance with a TAPPI Test Method, is usually one or the average of two or more such test determinations (see definitions in TAPPI T 400 "Sampling and Accepting a Single Lot of Paper, Paperboard, Containerboard or Related Product"). This practice allows the examination and possible elimination of suspect test determinations (from sets of 3 to 30 determinations) before the calculation of the final test result.
NOTE 2: This practice may also be applied to suspect test results (by substituting the words "test results" for "test determinations" throughout this document), when a laboratory must evaluate a large shipment requiring the determination and calculation of several test results.
Three categories of suspect determinations are considered:
A single suspect determination;
Two suspect determinations, one the least and the other the greatest in the set of replicate determinations; and
Two suspect determinations, the two largest or the two smallest in the set.