Exercise 7: Accuracy and precision

Exercise 7: Accuracy and precision

Origin of the error :Accuracy and precision • Systematic (not random) • bias • impossible to be corrected • accuracy • Random • can be evaluated by statistics • precision

Bias and precision Off-Base Model Silver Standard Hit or Miss Model Gold Standard Poor Precision Good Accuracy Good Precision Poor Accuracy Good Precision Good Accuracy Poor Precision Poor Accuracy

Accuracy Closeness of determined value to the true value. The acceptance criteria is mean value  15% deviation from true value. At LOQ, 20% deviation is acceptable.

Accuracy The accuracy is calculated using the following equation : The accuracy at each concentration level must be lower than 15% except a LOQ (20%)

Precision The closeness of replicate determinations of a sample by an assay. The acceptance criteria is  15% CV. At LOQ, 20% deviation is acceptable.

Precision Repeatability (r) Agreement between successive measurements on the same sample under the same conditions Reproducibility (R) The closeness of agreement between results obtained with the same method under different conditions

Precision… Considered at 3 Levels • Repeatability • Intermediate Precision • Reproducibility

Repeatability • Express the precision under the same operating conditions over a short interval of time. • Also referred to as Intra-assay precision • (within day)

Intermediate Precision • Between-run, between day repeatability … also named intermediate precision, which measures precision with time (often days) and may involve different analysts, equipment, reagents…

Precision: measurement • Should be measured using a minimum of 5 determinations per concentration • A minimum of 3 concentrations in the range of expected concentrations • The precision at each concentration should not exceed 15% except for the LOQ (20%)

Raw data

Precision: measurement • for a single measurement : CV(%) • for intra-day and inter-day precision • ANOVA

Variance of intermediate precision

Estimation of the two components of the variance • we need a proper statistical model that is the one-way nested analysis of variance model given as : and the variance of wij is . With i=1,2..n days and j=1,2,…replicates; in this equation, Yij are observed values, μ is the true value of the common stock solution being assayed in this experiment, bi is a random component for between-days variation and wij is another random component for within-day variation. The variance of bi is

ANOVA • Allows an estimation of the 2 variance terms • inter-day mean square (BMS) • intra-day mean square (WMS)

The exercise consist to compute these variances • By hand • Using an ANOVA (Excel) • With Analyse- it

The limit of quantification (LOQ) • LOQ is the lowest amount of analytes in a sample which can be determined with defined precision and accuracy • LOQ :  20%

Limit of quantification (LOQ) • The lowest standard on the calibration curve is the LOQ if: • no interference is present in the blanks at retention time of the analyte for this concentration • the response (analyte peak) has a precision of 20% and accuracy 80-120%

Three analytical areas LOD LOQ 2 3 1 Xb Area of quantification or CV<20% Area of detection not detected

Exercise 7: Accuracy and precision