ASSESSMENT AID:
The Basics of Decision Rules
What is a decision rule?
ISO/IEC 17025:2017 states that a decision rule is a “rule that describes how measurement uncertainty
is accounted for when stating conformity with a specified requirement” (ISO/IEC 17025:2017, p. 2). In
other words, when dealing with a piece of data that is just barely in or barely outside a tolerance limit,
a decision rule is a standardized way of deciding what side of the conformity decision that data point
ultimately falls on.
How do I know if this rule applies for my organization?
According to A2LA Accreditation Manager Rob Knake, “There are two very important elements of this
definition: ‘measurement uncertainty’ and ‘conformity with a specified requirement’. If you don’t have
either of those two elements then, by definition, you do not have a decision rule, or a decision rule would
not apply. The decision rule is the description of how we decide the acceptability of a statement, such
as ‘meets or does not meet specification’, ‘pass/fail’, ‘in/out of tolerance’. It explains how we have
evaluated the measurement decision risk associated with making that statement of conformity. It tells us
how much risk we feel is acceptable and how we evaluated the risk associated with that decision, so that
when we say something is good, we are reasonably sure it is good within some predetermined limits of
acceptable risk.”
Assuming this rule applies to me, what is the requirement per ISO/IEC 17025:2017?
Clause 7.1.3 of ISO/IEC 17025:2017 states that when a customer requests a statement of conformity, such
as “pass/fail” or “in/out of tolerance”, the vendor must clearly define the specification and the decision
rule that will be applied during the contract negotiation process with its customer, unless it is already
found in a specification or method. Furthermore, in clause 7.8.6.1 the decision rule must be applied and
documented, which typically occurs in the calibration report.
When should a decision rule be applied?
Decision rules are generally applied for cases when it is difficult to determine conformity with a specification
because the data point and its associated uncertainty is found near or outside a tolerance limit. (See Figure 1).
Figure 1
Upper Limit
Decision Rule
Needed
Lower Limit
Data Point
Uncertainty
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Decision rules may also be applied when the expanded uncertainty of the measurement spans a significant
portion of the tolerance limit (not pictured). Significance should be discussed with the customer. It is
not uncommon, especially in high measurement accuracy and precision conditions, e.g. μV, μm, μHz,
etc., where the expanded uncertainty spans the tolerance limit (Uexp = ± 1 μV; Tol = ± 1 μV). When this
occurs, it becomes increasingly important to have, document, implement and report on the decision rule
conversations with the customer.
How do I select a decision rule?
In many cases the decision rule may be found in a test or calibration method or specification. In other
cases, the customer dictates the decision rule. Often, customers who use manufacturers or third-party
calibration laboratories for service do not understand decision rules or their own acceptance requirements,
and it is incumbent upon the service providers to find a means to communicate and, in some cases,
educate the customer. Beyond this, it is up to the vendor how they determine the conformance to
specifications of these questioned data points. You may choose to always consider them “out of tolerance”
or “fail”, “in tolerance” or “pass” or something in between.
Do you have a simple means of helping me decide whether a decision rule applies for me?
We do! See Figure 2 for a flowchart on whether a decision rule must be defined, agreed to and applied.
Figure 2
Decision Rules
Did Customer Request a
Pass/Fail Statement?
YES
Does Uncertainty Come
Into Consideration?
NO
YES
STOP Worrying About
Decision Rules!
Do Customer or Method
Tell Lab How to
Incorporate Uncertainty?
YES
NO
Decision Rule Defined,
Agreed To, & Applied
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Can you provide an example of a decision rule?
Of course! Below are three examples of decision rules. Keep in mind that these are only
a few examples of a decision rule and that there many others that can be applied.
n Decision Rules found in Method or Specification:
ASTM E18 for Rockwell Hardness of Metallic Materials includes its own decision rules
n Example Decision Rules requested by the customer:
Measured Value ± measurement uncertainty ≤2.0% tolerance limit per ANSI/NCSLI Z540.3
n Example (Basic) Decision Rule created by the Test or Calibration laboratory:
Measured Value ± measurement uncertainty < tolerance limit (lower risk) or
Measured Valued ± measurement uncertainty = tolerance limit (higher risk)
For more information on decision rules and/or
ISO/IEC 17025:2017 consider registering for:
MS 111 – ISO/IEC 17025:2017 The New Standard for Laboratory Competence or,
for a more mathematical approach, consider:
EMU 401 - Measurement, Risk and Finance: Making Decisions on Conformance
Introduction to ISO/IEC 17025 for Technicians (e-Learning)
ISO 17025 Compliance: Uncertainty Management (e-Learning)
through
WWW.A2LAWPT.ORG
For more information on consulting service to assist in
developing decision rules please contact us at
www.a2lawpt.org/form/contact-us or info@A2LAWPT.org.
REFERENCES
ISO. (2017). ISO/IEC 17025:2017 International Standard: General
requirements for the competence of testing and calibration
laboratories, Switzerland.
Knake, Rob. (2019). Help! I can’t decide! A brief discussion on
decision rules. LinkedIn. Retrieved from www.linkedin.com/pulse/
help-i-cant-decide-brief-discussion-decision-rules-rob-knake/.
NCSL International. (2006). ANSI/NCSL Z540.3-2006 American national
standard for calibration – requirements for the calibration
of measuring and test equipment, Boulder, Colorado, USA.
ASTM International. (2019). ASTM E18-19 Standard Test Methods
for Rockwell Hardness of Metallic Materials, West Conshsohocken,
Pennsylvania, USA.
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