OMB Meeting Book_9-11-14

Accuracy -

Table 2

Reference value 1

SRM Product

Measured value

Code

Analyte

Bias T stat

p -value Confidence Interval

Conc. (mg/kg pwd)

(mg/kg pwd)

0.0420 0.0437 0.0443 0.0423 0.0430

1849a

Vitamin B12

0.0482

-14.483

0.00000051

0.0414

0.0433

-

0.0420 0.0419 0.0420 0.0401 1. Approximately 50% of the data used to determine the NIST vitamin B12 reference value were generated with the microbiological method which is known to be biased high.

T stat

=

where: = mean of the differences between measured results and reference value sd = standard deviation of the differences between results and reference value

n = number of replicate analyses of SRM

H 0 : = 0, The null hypothesis is that there is no difference between the measured results and the certified value:

p -value: is the probability (at the 95% levels of confidence) of obtaining the test statistic assuming that the null hypothesis is true. p -value < 0.05 indicate a difference of statistical signifance between measured results and SRM.

95% confidence interval: The true population of the measured value expected to fall within the confidence interval estimate 95% of the time.

lower limit = mean -

upper limit = mean +

where: = critical values of the t-distribution

sd = standard deviation of the differences between results an reference value n = number of replicate analyses of SRM

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