The following terms are used here and in literature elsewhere to discuss reference materials.

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There are 5 names in this directory beginning with the letter P.
Performance Gate
Synonymous with Control Limit
Physical Outlier
(as opposed to analytical outlier) - A single result or entire set of results deviating in either accuracy or precision from others in the set or from other sets, respectively, due to inhomogeneity of the reference material.
Pigeon Pair, Paired Offsets, Bracketed Standards, Variance Standards
Terms used to describe a pair of standards of almost identical matrix that differ in concentration of one or more certified elements by an amount approximating the typical measurement error of methods in common use at commercial analytical laboratories. All geochemists and geologists involved in QC are mindful of the shortcomings of insertion of the same standard or standards in batch after batch of samples submitted to their laboratory for analysis. After several months the laboratory’s QC manager soon becomes familiar with the control limits they are expected to report within for a particular standard. This knowledge can severely compromise the value of the standard. One strategy of overcoming this problem of familiarity is the production of paired offset standards (also referred to as pigeon-paired or bracketed standards). Instead of preparing one standard of a specific grade two are prepared at concentrations bracketing this. The concentration offset between the two is generally chosen to be of a magnitude comparable to measurement error of the analytical method employed (typically 1-2% for fusion XRF, 5% for ore grade precious and base metals and 10% for geochem grade precious and base metals). Obviously the preparation of standards from end-member components (e.g. barren and high grade material) lends itself well to the incorporation of paired offsets as they have compositional characteristics (both chemical and mineralogical) almost identical to one another.
parts per million (equivalent to grams per tonne). SI unit equivalents ≡ mg/kg ≡ µg/g ≡ 0.0001 wt.% ≡ 1000 ppb, parts per billion. 1 ppm ≡ 0.029 opt (troy ounces per short ton). Alternatively, 1 opt ≡ 34.2857 ppm.
The closeness of agreement between results obtained by measurement of the parameter numerous times under prescribed conditions.