AOAC SPIFAN ERP Meeting Book-March 16, 2016

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Accuracy ‐ Analytical bias compared to reference material Certified Reference Material

Table 2

Measured Results

T stat Bias

Difference from  certified value

Uncertainty Certified range Concentration

Confidence interval 

CRM product

Analyte

Certified value

p ‐value

Nominal

2.3

0.0

NIST 1849a

Folic Acid

2.293

0.062

2.231 ‐ 2.355

0.0

#DIV/0!

#DIV/0!

#NUM!

‐ #NUM!

Mean   

2.3

0.0

SD   

#DIV/0!

#DIV/0!

d

T stat =

n

/ sd

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 d

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

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

sd

sd

t crit

t crit

lower limit  = mean ‐

upper limit = mean +

n

n

crit t

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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