MARLAP Measurement Uncertainty Workshop

Presentation Transcript

51st Radiobioassay and Radiochemical Measurements Conference

Stateline, Nevada

October 24, 2005

Introduction

The Multi-Agency Radiological Analytical Protocols manual (MARLAP) was completed & released in 2004
Goal: Provide guidance and a framework to assure that laboratory radioanalytical data meet specific needs of a program or project

EPA

DOD

DOE

DHS

NRC

FDA

USGS

NIST

Kentucky

California

The MARLAP Working Group

The MARLAP Approach

Many have talked about the need for a “performance-based” approach to radioanalytical work
MARLAP offers guidance to implement it
Measurement uncertainty is a key concept in the approach

Uncertainty in MARLAP

Measurement uncertainty and MARLAP’s derived concept of “method uncertainty” appear throughout the manual
Measurement uncertainty defined as a parameter associated with the result of a measurement
Method uncertainty defined (in MARLAP) as a performance characteristic of a measurement process

Method Uncertainty

MARLAP defines it as

Predicted uncertainty of the result that would be obtained if the method were applied to a hypothetical laboratory sample with a specified analyte concentration

The specified concentration is typically the action level
Must specify sample characteristics and measurement conditions (as for MDA)

MARLAP’s Use of Uncertainty

MARLAP’s performance-based approach uses:

Method uncertainty in the selection and validation of a method (Chapter 6)
Method uncertainty in the continuing evaluation of a lab’s work (Chapter 7)
Method uncertainty and measurement uncertainty in data V&V (Chapter 8)
Measurement uncertainty in routine lab QC (Chapter 18)

Evaluation of Uncertainty

Most of MARLAP’s guidance for expressing and evaluating uncertainty is in Chapter 19

The “GUM”

In 1993 ISO published Guide to the Expression of Uncertainty in Measurement (the GUM)

Developed by 7 international organizations
Last revised (corrected) in 1995

Purposes:

Promote full information on how uncertainty statements are arrived at
Provide a basis for the international comparison of measurement results

The GUM (continued)

The GUM is a guide, which presents

Terminology
Notation
Methodology

Includes worked-out examples

Including a radiochemistry example

Recommendation

MARLAP’s primary recommendation regarding measurement uncertainty is

Follow the GUM

Rationale

If everyone follows the GUM:

All of us speak and write the same language about uncertainty
We can interpret each others results and uncertainty statements
Maybe clients can interpret our results and uncertainty statements

Additional Recommendations

MARLAP further recommends:

Labs should consider all sources of uncertainty
Evaluate and propagate all uncertainty components believed to be potentially significant
Always report the uncertainty
Always explain the uncertainty