Declaration of conformity under ISO 17025: decision rule and statement
A declaration of conformity is a laboratory’s statement that a measurement result is within or outside a specified requirement. Under ISO/IEC 17025 (clause 7.8.6), that statement must be based on a documented decision rule, which takes the measurement uncertainty into account. Without that rule, “conforms” or “does not conform” is a guess. The Dutch Accreditation Council (RvA) assesses whether your laboratory knows and applies the rule, within its scope of accreditation. A declaration of conformity is also a statement when you do not call it one. If you show results in bold or colour so that they appear to pass or fail, the RvA treats that as a declaration (INF002, clause 7.8.6). Use this page as a follow-on to the explanation of what ISO 17025 is.
It sounds technical, but the core idea is simple. A value never sits exactly on the limit. The decision rule determines what you do when the value is close to the limit.
What is a decision rule?
A decision rule is the agreement that sets out how you handle uncertainty when making a conformity statement. You choose it in advance, record it, and apply it consistently.
| Decision rule | What you do | Risk |
|---|---|---|
| Simple: compare the measured value with the limit | Pass if the value is within the limit, otherwise fail | A measurement that just falls inside the limit may in reality be outside it |
| With a safety margin (guard band) | Pass only if the uncertainty band lies entirely within the limit | Stricter: you reject more often, even when the product is good |
| With three outcomes | Conforms, does not conform, or cannot be determined if the uncertainty band straddles the limit | Requires more explanation to the client, but gives the most honest picture |
The choice is yours and your client’s, but it must be agreed in advance and written down. The auditor wants to see that the rule was applied to every statement. The international guide ILAC G8 describes these rules in detail. The RvA allows you to add or subtract the measurement uncertainty at the specification limit, provided you agree this explicitly and state it clearly in the report (INF002, 7.8.6 and 7.1.3).
The worked calculation
A fictional laboratory measures the thickness of a coating. The requirement is a maximum of 50 micrometres. The measurement gives 49.2 micrometres, with an expanded uncertainty of 1.0 micrometre (k=2, fictional).
Determine the uncertainty band
The true value lies, with about 95 per cent coverage, between 48.2 and 50.2 micrometres. The upper end of that band is above the requirement of 50.
Choose your decision rule
Suppose your client has agreed that you only say “conforms” if the whole band lies below the limit. Then the statement for this measurement is not a pass. If the rule has a third outcome, you write “cannot be determined”. Otherwise you write “does not conform”. The product has not been proven bad: you simply cannot approve it with the agreed level of confidence.
Record the statement and the rule
The certificate states the measured value, the uncertainty, the requirement, the chosen decision rule and the statement. Without those four elements, the statement cannot be checked.
If you change the decision rule to “pass if the measured value is below the limit”, your laboratory says “conforms”. That is a different statement for the same measurement. This is why the rule must be fixed in advance. All values in this example are fictional.
What must appear on the report?
A conformity statement on a report should be traceable. The reader must be able to see what the statement is based on.
- The measured value and the uncertainty, with the coverage factor used. For a testing laboratory, the statement about uncertainty must appear explicitly in the report if the uncertainty affects whether the limit is met (INF002, 7.8.3.1).
- The requirement or specification against which the test was carried out, with its source.
- The decision rule, in one sentence or with a reference to the procedure.
- The statement itself, and where relevant the risk you accept.
- The date and the person responsible.
Note: a laboratory that makes a conformity statement without naming the requirement gives the client a number without context. This is a common mistake.
A conformity statement based only on the nominal value ignores measurement uncertainty. For a measurement close to the limit, that can mean a product is wrongly approved or rejected.
Common mistakes
- No decision rule recorded. The rule exists only in the analyst’s head.
- Changing the rule per job. A client who asks for “a bit more lenient” is a reason to discuss the rule formally, not to change it quietly.
- Uncertainty not taken into account. The statement is then based on a number without a band.
- A statement without a requirement. A pass or fail without the requirement cannot be checked.
How do you record it in your procedure?
A short work instruction is enough. Include four points.
- Which decision rule you use, per type of requirement or per client.
- How you take the measurement uncertainty into account when comparing with the limit.
- What you put on the report, and in which order.
- Who may make the statement, and who checks it.
It is also worth discussing with the client in advance which decision rule belongs to their requirement. Many clients have no preference of their own, so you can choose the rule together.
Frequently asked questions
Must every laboratory make a conformity statement? No. Only when the client or the specification asks for one. Without a statement, you only provide measurement results. Note, however, that showing results in bold or in colour can count as a statement.
Is the strictest decision rule the best? Not automatically. A strict rule lowers the risk of wrongly approving a product, but raises the risk of wrongly rejecting it. Choose based on what the client needs.
What if the client does not want to choose a decision rule? Then your laboratory should propose a rule and document the choice. Preferably ask the client to confirm it in writing.
Does this also apply to calibration? Yes. For calibration, the question is often whether the measuring instrument is within tolerance. On the example certificate in ASR106, a declaration of conformity must state which results it applies to, which specifications are or are not met, and which decision rule was used. See also calibration laboratory.