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What is known...

Decision Rules for Proving
Conformity or NonConformity
with Specifications

Relationship with the uncertainty of measurement
ISO 14253-1, which is a part of "Geometrical Product Specification GPS", provides "Rules for establi-
shing the conformity or nonconformity with specifications". These rules are valid
for "Inspection by measurements of workpieces and measuring equipment".
This ISO standard makes allowances for the uncertainty of measurement – or more precisely for the
true uncertainty of any measurement whenever the conformity or nonconformity with a given specifi-
cation must be proved. So, for a workpiece, the specification matches a preset tolerance while being
equal to the maximum permissible errors for a metrological characteristic (MPE) for a measuring
instrument.
Any given specification is a constant, whereas the measurement uncertainty is a variable which is
affected by many components. Therefore, the zone of conformity or nonconformity depends on the size
of the effective expanded uncertainty U.

Rule for proving conformity Lower specification limit LSL Specification zone Upper specification limit USL
Conformity is proved when the (= lower tolerance limit) (= Tolera0nce zone) (= upper tolerance limit)
measurement result y is lying within the
specification zone, reduced on either or lower maximum permissible or upper maximum permissible
side by the expanded uncertainty U. error MPE error MPE
Consequently, workpieces or measuring Deviations
instruments can be accepted as far as Two-sided specification
their conformity with the specification is
proved by the manufacturer (supplier). UU UU

Rule for proving nonconformity Nonconformity zone Conformity zone Nonconformity zone
Nonconformity is proved when the mea-
surement result y is lying beyond the spe- Uncertainty Range Uncertainty Range
cification zone, increased on either side
by the expanded uncertainty U. In such a Specification limit LSL or USL
case, the relevant measuring instruments (= Tolerance limit) or
can be rejected if the purchaser (cus-
tomer) gives evidence of its non-confor- 0 maximum permissible error MPE
mance. Specification zone
(= Tolerance zone)
Neither conformity nor nonconfor-
mity can be proven One-sided specification Deviations
This often happens when the mea-
surement result y associated with the UU
expanded uncertainty U includes either
of the LSL or USL specification limits. Conformity zone Nonconformity zone
As a result, workpieces or measuring
instruments can neither be automatic­ ally Uncertainty Range
accepted nor rejected.
For such "dead end cases", it is advisable Lower specification limit LSL Upper specification limit USL
to follow the rule below. (= lower tolerance limit) (= upper tolerance limit)
– Repeat all measurements based on a
reduced uncertainty, so that conformity or lower maximum permissible or upper maximum permissible
or nonconformity can clearly be demons- error MPE error MPE
trated. Usually, proceeding in this way
benefits to the party that’s able to provide Specification zone
the needed proof. (= Tolerance zone)
– Come to a mutual agreement providing
the procedure to be applied if such cases Deviations
arise.
Conformity with
specification is proved

U U
Result of
measurement y

Nonconformity UU
with specification Result of
measurement y
is proved

Neither conformance UU
nor nonconformity Result of
with specification measurement y
can be proved

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