On Thu, 23 Apr 2009 15:29:04 -0700, Jim Kelley
wrote:

Cecil Moore wrote:
Jim Kelley wrote:
... and as any good dry labber knows, it's a dead giveaway to report a
precision greater than one can actually measure. :-)

I have reported no precision - my 100 MHz scope has
not been calibrated since I retired.

Precision is the number of sig figs. You "might" have calculated three,
rounded up, and reported two.

Precision is NOT accuracy. Resolution is NOT precision. Accuracy is
defined with precision to a resolution.

You can state a value with great precision and be 100% in error.

100 V is quite precise; "about" 100 V is less precise.

100 V has three places of resolution.

If the true value is actually 201.45 V then 100 V is precise, somewhat
resolved, but inaccurate. On the other hand, 201.45 V is very
precise, highly resolved, and accurate to within 0.005 V (if we are to
trust it as a reference) or 25 parts per million.

I can anticipate the objection (to confound my statement above) that
100 V has both resolution and precision. True, but that objection
would miss the point. Some standards are nominal (or cardinal) values
such as an 1 MHz URQ-23 frequency standard: 1 place of resolution, but
highly precise with an accuracy of (from my experience) of 6 parts per
trillion (after calibration against a cesium beam standard).

I can anticipate the fine objection that the nominal value of 1 is
actually 1.00000000000. Again, true, but in a world where you own an
URQ-23 (and no one has access to HP 5071 cesium clocks), then you get
to snub that objection and demand: "It IS exactly 1!"

73's
Richard Clark, KB7QHC