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HAVING CONFIDENCE IN SPECIFICATIONS
David Deaver
Fluke Corporation
PO Box 9090
Everett, WA 98206
425-446-6434
[email protected]
Abstract: This paper presents some of the unique specification would have to be assigned
challenges in creating instrument to each instrument, taking into account its actual
specifications from a manufacturer's performance and the operating environment. In
perspective and some of the difficulties practice, this is done by many laboratories when
using specifications as an estimate of characterizing a standard but is seldom done by
uncertainty from a user's perspective. the manufacturer.
Despite the difficulties, specifications are the
most common means of determining the Manufacturing Yield Targets
Type B uncertainty for the standard when
making a GUM (Guide to Uncertainty of Though specifications may be stated with a
Measurement) compliant uncertainty confidence, typically between 95% and 99.7%,
analysis. An explanation is presented for the all the points tested on the production line must
practice of providing 95% and 99% be measured within the specification limits
confidence specifications, warranting only before the product can be shipped. For a
the 99% specifications, and verifying the 99% standard resistor, whose value is specified at a
specifications to 95% confidence. 95% confidence level to be within limits centered
about the nominal value for the resistor, we
THE BASIS OF SPECIFICATIONS could expect to have to re-trim 5% of the
standards before they could be shipped.
Simply stated, specifications are an implied Specifying a complex instrument at the true 95%
contract between a buyer and seller of a piece of confidence level for each point would be a
equipment. The seller expects to be paid if the manufacturing disaster. For example, each
product performs within its specifications and the Fluke Model 5520A Multiproduct Calibrator is
buyer expects to receive an instrument which tested at 552 points on the production line prior
lives up to the promises made in the data sheet. to shipment. If each of the points has a 95%
Specifications, however, may or may not be a probability of being found in tolerance, there
552
very good representation of the product's actual would only be a 0.95 = 0.00000000051%
performance. The manufacturer considers a chance of finding all the points within the
number of factors when establishing the specification limits if the points are independent!
specifications: Even if we estimate 100 independent points
(about 2 per range for each function), we would
100
Actual Instrument Performance still have only a 0.95 = 0.6% chance of being
able to ship the product. Figure 1 shows the
One would hope there is some correlation probability of an instrument passing all points
between the actual performance of an tested if the points are independent and if all
instrument and its specifications. Ideally, the points have normal probability density
specification would correspond exactly to the distributions with the same confidence interval,
uncertainty. To approach the ideal, however, a 95%, 99%, or 99.7%.
Competitive and Market Needs
1E+00
99.7%
1E-01
If there is not a compelling need to make
1E-02
specifications as tight as absolutely possible,
1E-03
99% there is considerable benefit to the manufacturer
1E-04
to keep them loose in order to enjoy high yields
1E-05
95%
and low warranty costs. Similarly, for the user, it
1E-06 is nice to have higher confidence the unit is
1E-07 performing within the specification limits and
1E-08 0 200 400 600 800 1000
enjoy long calibration intervals. Often, however,
1E-09 manufacturers have specsmanship battles with
1E-10
Num ber of Independent Points Tested (N)
their competitors, each wanting to have the
better specifications. And users ask for tighter
Fig. 1 Probability of all N Points In-Tolerance specs on test equipment to maintain low
uncertainties and high TURs (Test Uncertainty
For our example of 100 independent points, on
Ratios) for their calibrations. As we have seen
average, we would have to design the individual
however, along with tighter specifications comes
points to a 99.95% confidence level to have 95%
the adverse impact on the yields, warranty
confidence that the instrument would pass the
costs, and percent-in-tolerance numbers.
complete verification test. Table 1 lists the
confidence levels for individual points (P)
required for an overall instrument yield (Y) of Environmental Conditions
95%, 99% and 99.7%.
Specifications for an instrument must take into
N Y=95% Y=99% Y=99.7% account its operating environment. Though most
instruments are specified for a broad
1 0.95 0.99 0.997 temperature range such as 0-50