As an example in the case of a silver 8’ FaroArm we report single point +/- 2 sigma repeatability of +/-0.003”.
The expected +/-(max-min)/2 would be approximately +/- 3 Sigma or +/- 0.0045”. Another way to express this
Issue #: 21 Revised: 8-28-02 Page 2 of 5 f:\control\referenc\06servic\english\06ref176-003.doc
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is to state the total error band for a single point of 0.009”. We chose the +/- terminology since the true value is
at the middle of the band and therefore the greatest deviation from the true value is 1/2 of the band.
The accuracy of a linear displacement measurement is the sum of the deviations of the single point. The
standard deviation is the square root of the variance. Therefore, if the variance is the same for each of any two
points, the standard deviation of the linear displacement is the square root of 2 (or 1.414) times the single point
standard deviation. Therefore the displacement +/- 2 sigma is +/- 0.0042 and the displacement +/- 3 sigma is
.0063” , or total band of 6 sigma .0127”.