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Keysight Technologies
A Guard-Band Strategy for
Managing False-Accept Risk
White Paper
Abstract
When performing a calibration, the risk of incorrectly declaring a device as in-tolerance (false-
accept risk) is dependent upon several factors. Those factors include the specified tolerance
limit, guard-band, the calibration process uncertainty and the a priori probability that the device
is intolerance. A good estimate of the a priori probability may be difficult to obtain. Historical or
device population information for estimating the a priori probability may not be readily available
and may not represent the specific device under test.
A common strategy for managing measurement decision risk is to choose a guard-band that
results in the desired false-accept risk given the tolerance limit, the calibration process uncer-
tainty and the a priori probability. This paper presents a guard-band strategy for managing false-
accept risk with only limited knowledge of the a priori probability that a device is intolerance.
Introduction
When determining if measurement quantities are within specified tolerances, ANSI/NCSLI Z540.3-2006 specifies that
the maximum level of false-accept risk be no more than 2%. False-accept risk is the probability that measuring an out-of-
tolerance device will indicate an intolerance condition due to measurement error. False-reject risk is the probability that
measuring an in-tolerance device will indicate an out-of-tolerance condition due to measurement error. False-accept and
false-reject occurrences have financial consequences, and therefore, minimizing both is often a worthwhile objective.
One strategy for managing false-accept risk is to apply a guard-band such that the acceptance limits are more stringent
that the tolerance limits. A common practice (see [3]) is to set the guard-band to a value equal to the 95% expanded
uncertainty of the calibration process. This level of guard-band guarantees the Z540.3 false-accept risk requirement and is
attractive in that it only requires information that many calibration organizations routinely manage (that is, the tolerance
limits and the 95% expanded uncertainty, which is set as the guard-band). However, when using a guard-band to reduce
false-accept risk, a corresponding increase results in the false-reject risk. With the guard-band set to the 95% expanded
uncertainty, the false-reject risk can be disproportionately high (see Figure 3).
An alternative to applying a guard-band equal to the 95% expanded uncertainty is to determine the false-accept risk and
set an appropriate guard-band, if necessary, that adjusts the false-accept risk to the desired level. To determine the level
of false-accept (or false-reject risk) for a calibration measurement, the following information is necessary: