AOAC STANDARDS & METHODS ORIENTATION E-BOOK

9/10/2019

Precision

• The closeness of agreement between independent test results obtained under stipulated conditions. • Same lab, same samples, same analysts, same instrument.

• Expressed as:

– RSD(r) or RSD r

or as “RSD little r”

– RSDr estimated as 2/3 of RSDR

Table 4: Expected Precision (repeatability) as a Function of Analyte Concentration

Analyte % Analyte Ratio

Unit

RSD%

100

1

100%

1.3 1.9 2.7 3.7 5.3 7.3

10

10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9

10%

1

1%

0.01

0.1%

0.001

100 ppm (mg/kg) 10 ppm (mg/kg) 1 ppm (mg/kg) 100 ppb (μg/kg) 10 ppb (μg/kg) 1 ppb (μg/kg)

0.0001 0.00001 0.000001 0.0000001 0.00000001

11 15 21 30

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