FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 1 of 7

For the most current and official copy, check QMiS.

Sections in This Document

1. Purpose ....................................................................................................................................1

2. Scope .......................................................................................................................................1

3. Responsibility............................................................................................................................1

4. Background...............................................................................................................................2

5. References ...............................................................................................................................2

6. Procedure .................................................................................................................................2

6.1. Categories of Tests........................................................................................................2

6.2. Estimating Uncertainty ...................................................................................................3

6.3. Reporting Uncertainty Estimates....................................................................................5

7. Glossary/Definitions..................................................................................................................5

8. Records ....................................................................................................................................6

9. Supporting Documents .............................................................................................................6

10. Document History .....................................................................................................................6

11. Change History.........................................................................................................................7

12. Attachments..............................................................................................................................7

1. Purpose 

This procedure describes the process for the estimation of measurement 

uncertainty (MU).

2. Scope

These procedures apply to regulatory testing carried out by the Office of 

Regulatory Science (ORS) laboratories.

3. Responsibility 

ORS laboratories are responsible for the collection of data and the 

determination of the estimated uncertainty for analytical methods in use.
FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 2 of 7

For the most current and official copy, check QMiS.

4. Background 

Every measurement or test has an error of measurement. If repeated, a test or 

measurement often gives a different result, even though it usually is very 

similar to the original result. Therefore, a test or measurement gives only an 

approximation of the true value of the quantity to be measured. A 

measurement or test is only complete if it includes the measurement 

uncertainty of the test. This can be thought of as a quantitative indication of 

the quality of the result.

Defining the measurement uncertainty is the same as determining accuracy of 

results.

5. References 

A. National Institute of Standards and Technology (NIST). Technical Note 

1297, Guide for Evaluating and Expressing the Uncertainty of NIST 

Measurement Results.

B. EURACHEM. CITAC Guide, Quantifying Uncertainty in Analytical 

Measurements. 

C. PL-3 Policy on Uncertainty Measurement, Perry Johnson Laboratory 

Accreditation

D. ISO/IEC 17025:2017, General Requirements for the Competence of 

Testing and Calibration Laboratories. Section 7.6

E. AOAC International Guidelines for Laboratories Performing 

Microbiological and Chemical Analysis of Food, Dietary Supplements, 

and Pharmaceuticals – An Aid to Interpretation of ISO/IEC 17025:2017; 

August 2018.

6. Procedure

6.1. Categories of Tests

A. Each laboratory is required to identify and record the measurement 

uncertainty category (listed below) for each method on the scope of 

accreditation. Additionally, each laboratory is required to list all the 

components of uncertainty and estimate the uncertainty for all 

quantitative methods or technologies on their scope of accreditation. 

B. The three different categories of tests as well as the extent of 

uncertainty estimation are:
FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 3 of 7

For the most current and official copy, check QMiS.

1. Qualitative tests and tests reported on a categorical or nominal 

scale. These do not need a measurement of uncertainty; however 

where possible the laboratory should identify possible areas for error 

in order to control these factors. 

2. Well recognized test methods that specify limits to the values of the 

major sources of uncertainty of measurement and specify the form 

of presentation of calculated results. In such cases, the laboratory is 

considered to have satisfied the uncertainty requirements by 

following the method and reporting instructions; however when 

verifying the method the laboratory should confirm their MU 

compares with the established MU for the method.

3. If a new quantitative test method does not specify or define a 

measurement uncertainty, the laboratory must determine the 

method’s measurement uncertainty. Uncertainty can be estimated 

using laboratory control samples or the root sum square (RSS) 

method. After the application of the required calculation or 

uncertainty estimation technique provided in this procedure, other 

calculations and techniques can be used as supporting evidence.

6.2. Estimating Uncertainty

The steps for estimating uncertainty are discussed below. Measurement 

uncertainty does not usually include uncertainty due to sampling or bias.

A. Steps using relative standard deviation of laboratory standards/control 

samples run through all method steps. 

1. Perform spiked determinations at different concentrations including 

tolerance limit.

2. Calculate concentration and percent recovery.

3. Calculate the standard deviation (SD) and relative standard 

deviation (RSD) on results where the process is in statistical control 

(no outliers or out of control results).

4. Calculate the measurement uncertainty at the 95% confidence level 

as follows:

U = k X RSD

where:

U = uncertainty

k = coverage factor (for 95% and 50 points, use 2; for less than 50 

points, use the appropriate t statistic for 95%)
FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 4 of 7

For the most current and official copy, check QMiS.

5. To calculate the measurement uncertainty interval for a measured 

value, calculate as follows:

Interval = U X c

where:

c = concentration

B. Steps using the root sum square (RSS) method.

1. Clearly define what is being measured.

2. Review the method and identify every possible source of 

uncertainty. Sources include, but are not limited to:

a. Reference standards and reference materials

b. Methods and equipment used

c. Environmental conditions

d. Properties and condition of the unit under test

e. Operator

3. Review the sources and determine whether or not the components 

are included when running laboratory control samples.

4. Quantify all the components. Estimates can come from a variety of 

sources and do not need to be overly rigorous. Possible sources of 

quantitative estimates include: 

a. method validation studies, 

b. information from published methods or textbooks,

c. calibration certification, 

d. manufacturer’s specifications, and 

e. experience.

5. Consider the components. Assume that the components are 

independent. Every source may not have to be evaluated if they are 

deemed insignificant. Components that are less than a fifth of the 

largest component can be eliminated.

6. Combine the components. The usual method is to square all 

independent components, add them, and then take the square root 

of the sum. This is called the root sum square method and gives the 

combined standard uncertainty.
FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 5 of 7

For the most current and official copy, check QMiS.

7. Expand the combined standard uncertainty. Multiply the combined 

uncertainty by a coverage factor based on the level of confidence 

needed. In most cases a confidence level of 95% is sufficient, so k 

=2. If 99% confidence is required, then k =3. When estimates are 

based on limited data, k may vary according to the student t 

distribution. For example, if the estimate is based on analyses of 20 

laboratory control samples, the coverage factor of k = 2.1 for 95% 

confidence.

C. Measurement Uncertainty budgets are periodically reviewed and 

updated to reflect changes in the laboratory, equipment, procedures, or 

personnel.

6.3. Reporting Uncertainty Estimates

A. The extent of reporting of the estimates of uncertainty depends on the 

needs of the client, the specifications for the test, and the intended use 

of the result. Measurement uncertainty is not a major contributor in 

basing regulatory actions since the FDA will err on the side of public 

safety when making those decisions. 

B. MU estimate is not reported unless specifically requested by the 

customer. Nonetheless, the estimate is to be calculated, along with the 

means of making the estimate. Records are sufficient to allow 

replication of the calculation.

C. If reported, the uncertainty is reported to the same number of significant 

figures as the result. In most cases, two significant figures suffice. The 

estimate should also state the level of confidence associated with the 

coverage factor. Finally, the uncertainty is reported in the same units as 

the result.

D. The bias is reported in addition to the uncertainty when the method has 

a known bias and the bias adjustment was not performed.

7. Glossary/Definitions 

A. Coverage factor – The coverage factor is the number that is multiplied 

by the standard uncertainty to produce an uncertainty estimate that will 

contain a large fraction of all values that might be obtained on a test. 

The coverage factor is commonly noted as k, and k=2 is used for 95% 

coverage, and k=3 for 99% coverage.
FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 6 of 7

For the most current and official copy, check QMiS.

B. Expanded uncertainty – The expanded uncertainty is the combined 

standard uncertainty (or standard uncertainty, if there is only one 

component), multiplied by the coverage factor.

C. Measurement uncertainty – The measurement of uncertainty is the 

parameter associated with the result of a measurement that 

characterized the dispersion of the values that could be reasonably 

attributed to the measurand.

D. Measurand - The quantity being measured is the measurand (i.e. the 

concentration of an analyte). 

E. Repeatability conditions – Identical samples prepared at the same time, 

by the same analyst, under identical conditions, run on the same 

instruments are repeatability conditions.

F. Reproducibility conditions – Reproducibility conditions are identical 

samples analyzed under different conditions, including any of the 

following: different times, different equipment, different analysts, or 

different laboratories.

G. Standard uncertainty – The standard deviation for uncertainty is either 

for the test or for a component of the test. It can be expressed in the 

units of the measurement, or a percentage, but all components are 

expressed in the same terms before they can be combined.

8. Records 

A. Measurement uncertainty determinations, calculations and values for 

laboratory methods in use

9. Supporting Documents 

B. ORA Laboratory Manual, Volume II, ORA-LAB.5.4.5 Methods, Method 

Verification and Validation

10. Document History

Revision

#

Status*

(D, I, R) Date Author Name and Title Approving Official Name and Title

1.2 R 03/23/2005 LMEB LMEB
FOOD AND DRUG ADMINISTRATION

OFFICE OF REGULATORY AFFAIRS

ORA Laboratory Manual Volume II

Document Number:

ORA-LAB.5.4.6

Revision #: 02

Revised:

05/15/2019

Title: 

Estimation of Measurement Uncertainty Page 7 of 7

For the most current and official copy, check QMiS.

02 R 05/15/2019 LMEB LMEB

* - D: Draft, I: Initial, R: Revision

11. Change History

Revision 

# Change

02 Revisions made to align this procedure with new ISO/IEC 17025 and AOAC 

requirements. Revision to formatting and policy clarifications were also made.

12. Attachments

None