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Method Verification
The HowM.L. Jane Weitzel ALACC Chair
The What
Released early 2008
http://www.aoac.org/alacc_guide_2008.pdf
Meet ISO 17025 Requirement
Categories of Methods
The six categories of chemical analytical methods are:
1. Confirmation of Identity, a method that ensures a material is what it purports to be or confirms the detection of the target analyte.
2. Quantifying an analyte at a low concentration. 3. Determining if an analyte is present above or
below a specified, low concentration (often called a Limit Test). The specified concentration is close to the LOQ.
Categories of Methods
4. Quantifying an analyte at a high concentration.
5. Determining if an analyte is present above or below a specified, high concentration (often called a Limit Test). The specified concentration is substantially above the LOQ.
6. Qualitative test.
Tables 2 -6
Requirements of Method Verification for the Six Categories of Chemical Test Methods (Tables 2–6)
Category 1: Confirmation of Identity
ISO Technical Specification ISO/TS 21748, Guidance forthe use of repeatability, reproducibility and trueness estimates in measurement uncertainty estimation
ISO/TS 21748Template for Verification ISO 21748 is a thorough guide to verifying
a method. The examples will be based on ISO 21748
“Specification for the Method”
The results of collaborative study yield performance indicators (sR, sr) and, in some circumstances, a method bias estimate, which form a “specification” for the method performance.
In adopting the method for its specified purpose, a laboratory is normally expected to demonstrate that it is meeting this “specification.”
Repeatability
In most cases, this is achieved by studies intended to verify control of repeatability and of the laboratory component of bias, and by continued performance checks (quality control and assurance)
Many added benefits
Method verification Estimate of Uncertainty Better understanding of the method
Equation
y= μ +δ +B+ Σcix′i +e
Equation Components
y = µ + δ + Β + e + Σ cix′i
Result = ideal result + method
bias + lab bias + repeata
bility +sum of effects NOT included in collabora-
tive study
What if your method was not collaboratively studied?
The approach described by ISO 21748 is still relevant. Intermediate precision could be substituted for
reproducibility Fewer variables would be included in intermediate
precision study than during a reproducibility study There would be more effects included in Σ cixi’ The approach provides a detailed, organized procedure for
identifying and evaluating these effects
Method Bias
δmethod bias
Collaborative StudyNote: the study may not include this
component
Lab Bias
Β
lab bias
Collaborative StudysL
2
sR2 = sL
2 +sr2
Thus
sL2 = sR
2 - sr2
Repeatability
e
repeatability
Collaborative Studysr
2
Effects NOT includedΣ cix′i
sum of effects NOT included in collaborative study
Examine the equation and procedure.
Table Summary
y µ δ Β e Σ cix′i
Result Ideal result Method bias Lab bias Repeatability
Sum of effects not included
in collaborative
study
Source of Specification
for the method
Collaborative Study. Note:
the study may not include
this component
CollaborativeStudysL2
sR2 = sL
2 +sr2
ThussL
2 = sR2 - sr
2
CollaborativeStudy
sr2
Examine the equation and
procedure
Method Bias
δmethod bias
Collaborative StudyNote: the study may not include this component.
The study may include method bias, if for example the results are corrected for a known method bias. The procedure for assessing method bias is included in
the ISO standard in detail in “Incorporating Trueness Data”.(Uncertainty associated with CRV becomes important)
Repeatability - Equation
erepeatability
Collaborative Study sr2
Do repeatability study in lab. Calculate sw.Compare to sr
2 using the F test.
Bias
Often “accuracy” must be verifiedTerminology – bias
Can use same data from repeatability If Certified Reference Material is available
Lab Bias
Β
lab bias
Collaborative StudysL
2
sR2 = sL
2 +sr2
Thus
sL2 = sR
2 - sr2
Bias Specification
Follows ISO Guide 33 Δ = m - µ = mean - CRV
|Δ| < 2σD
|Δ| < 2√ (sR2 - sr
2 + sw2/n)
sL2 = sR
2 - sr2
n
Choose n so that the uncertainty of the bias is not significant
choose n such that the uncertainty √sw
2/n < 0.2sR
Good rule of thumb uncertainties less than 0,2 sR lead to changes of
under 0,02 sR in the overall uncertainty estimate.
Σ cix′i Look at equation to identify any source of
uncertainty that was not included in the collaborative study.
It is often convenient to consider each of the three factors the sample, the laboratory and the method when identifying gross uncertainties
Specificity
If your sample is not identical to those included in the collaborative study or method validation, you must assess the impact of the differences.
AOAC Food triangle can be useful
Useful Table 8 in ALACC Method Verification Guide Lists Parameter The difference from the validated method Required Equivalence Study
Useful Table 8
Table 8 Example
Uncertainty
For a collaboratively studied method and if you follow the process from ISO 21748, you will also end up with an estimate of the uncertainty for the method.
Table Summary
y µ δ Β e Σ cix′i
Result Ideal result Method bias Lab bias Repeatability
Sum of effects not included
in collaborative
study
Source of Specification
for the method
Collaborative Study. Note:
the study may not include
this component
CollaborativeStudysL2
sR2 = sL
2 +sr2
ThussL
2 = sR2 - sr
2
CollaborativeStudy
sr2
Examine the equation and
procedure
Examples