MARLAP Measurement Uncertainty Workshop

51 st 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

The MARLAP Working Group

 EPA  DOD  DOE  DHS  NRC  FDA  USGS  NIST  Kentucky  California

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

Purpose of This Workshop

  Explain uses of uncertainty in MARLAP Reiterate MARLAP’s major recommendations regarding uncertainty  Offer approaches and software tools to make uncertainty evaluations easier

Outline of the Workshop

   Part I – Uncertainty in MARLAP    Introduction 8:00–8:10 Overview of the GUM 8:10–8:40 Method Uncertainty 8:40–9:50 Break 9:50–10:10 Part II – Software tools  Kragten Spreadsheet Method 10:10–11:20  GumCalc: Measurement Modeling Software 11:20–12:00