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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