A laboratory report can contain both a measurement and a judgment. The measurement describes an observed quantity; a statement such as “pass” or “conforms” compares a result with a specification. Between those two steps sits a decision rule. Understanding that rule helps explain why a number below a limit does not always receive a passing statement.
What a decision rule adds
A specification sets a requirement. A decision rule explains how the laboratory accounts for measurement uncertainty when deciding whether that requirement has been met. The rule therefore connects the reported value, the specification and the conditions for declaring conformity.
Eurachem’s 2026 quality guide, section 26, describes this connection and the competing risks of accepting something that does not conform or rejecting something that does. It also explains that a conformity statement should identify the results concerned, the specification and the decision rule used.
These are reporting questions, distinct from how a sample was collected. The separate article on batch sampling and measurement uncertainty explains why a laboratory result may have a narrower reach than readers expect.
One result, two acceptance conditions
Consider a fictional test of an inert material, with an upper specification of 10 units. The laboratory reports 9.7 units and an expanded measurement uncertainty of 0.5 units. These invented numbers illustrate reporting logic only. They are not a cannabis, legal or safety threshold.
Under a simple acceptance rule, a result at or below 10 units meets the acceptance condition. The reported 9.7 therefore receives a passing statement under that rule.
Now imagine a different, explicitly defined rule: acceptance requires the result plus its expanded uncertainty to be at or below 10 units. Here, 9.7 plus 0.5 equals 10.2. The result does not meet that acceptance condition. This does not prove that the material’s true value exceeds 10 units. It shows that the evidence does not satisfy the stated condition for acceptance.
The measurement has not changed. The boundary for making the judgment has. A report should make clear whether an unsuccessful acceptance condition leads to a failure statement or another defined outcome.
Why a guard band changes the judgment
ILAC G8 guidance distinguishes simple acceptance from approaches using a guard band, which separates an acceptance limit from the specification limit. A guard band can reduce a particular decision risk while increasing another. The chosen approach needs documentation; where a rule is prescribed, that rule governs the statement.
The example above uses the full expanded uncertainty as its guard band, but that is one illustrative choice, not a universal formula. Uncertainty alone does not establish the probability of a wrong decision. Such a calculation needs additional assumptions and information. Changing rules after seeing an inconvenient result also prevents a consistent comparison.
A number beside a limit is not always a verdict
A report may show a measured result and a specification without explicitly declaring conformity. European Accreditation’s reporting FAQ distinguishes that presentation from a conformity judgment and notes that visual signals, including colour markers, may communicate such a judgment. A green cell can therefore deserve the same scrutiny as the word “pass.”
Accreditation is a separate consideration. As discussed in laboratory accreditation, scope and safety claims, the scope of recognised laboratory activities matters. An accreditation reference does not explain every conclusion a reader might draw from a report.
For comparisons between reports, matching units and test descriptions is only the starting point. Different acceptance conditions can make identical numerical results appear inconsistent even when both laboratories have applied their stated rules correctly.
A short report-reading checklist
- Which measured result and material does the statement cover?
- Which specification, version and units provide the comparison?
- Is the decision rule named or described clearly?
- Does the rule use uncertainty, and how?
- What do labels, colours and borderline outcomes actually mean?
A pass/fail statement is most informative when its boundaries are visible. It addresses a defined requirement under a defined rule. Extending that judgment to other properties, an entire batch or a general assurance of safety requires evidence beyond the label itself.