Gene:
That's all very interesting, except that we were not talking about
calculating the precision. Rather, we were talking about the degree of
precision implied by the use of the extra decimal digit.
Many many years ago, I earned a diploma in statistical quality control (and
was a member of ASQC), so I'm certainly familiar with the calculations you
describe. In fact, I learned them even before that, in high school physics.
<g>
Bill Potts, CMS
Roseville, CA
http://metric1.org [SI Navigator]
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On
> Behalf Of Gene Mechtly
> Sent: July 16, 2001 17:02
> To: U.S. Metric Association
> Cc: U.S. Metric Association
> Subject: [USMA:14436] Precision vs Accuracy
>
>
> On Mon, 16 Jul 2001, Bill Potts wrote:
> > ...
> > One need go no further than Webster's New Universal Unabridged
> Dictionary.
> > ...
> Webster's definitions are insufficient for calculating the precision
> and determining the accuracy of experimental data. Consider the
> following:
>
> Calculation of *precision* results from the analysis of *random errors*
> in a set of measurements of the same quantity under circumstances which
> are held as nearly constant as possible. Standard deviation (root mean
> square deviation from the mean value) is the usual measures of
> the precision
> of the set of observations.
>
> Accuracy is a determination or estimate of *systematic errors* (the
> closeness (lack of deviation) of the mean value of a set of measurements
> from a reference value established by better quality instruments, better
> techniques, and usually by better qualified observers or laboratories.
> Accuracy is ofter only an estimated value because a better reference value
> is not readily available, or does not exist.
>
> A set of measurements can be of good precision but poor accuracy.
> (e.g. tightly clustered measured values with small standard deviation, but
> having a mean value widely in error compared with a more correct value)
>
> Another set of measurements can be of good accuracy but poor precision.
> (e.g. widely scattered measured values but having a mean value with little
> deviation from a more correct value)
>
> Try making a scatter graph of twenty hypothetical observed values which
> illustrate good precision but poor accuracy; and a scatter graph of
> twenty values which illustrate poor precision but good accuracy.
>
> This is an easy exercise for a person who understands the difference
> between precision and accuracy!
>
> Gene.
>
