Factor that is known to have no effect on the response, used in an experimental design, to estimate repeatability standard deviation.
Example: A factor having levels "+" singing the first verse of the National Anthem at the experiment, and "−" singing the second verse of the National Anthem at the experiment.
Source:
PAC, 2016, 88, 407. 'Vocabulary of concepts and terms in chemometrics (IUPAC Recommendations 2016)' on page 414 (https://doi.org/10.1515/pac-2015-0605)