[ntpwg] Timing Definitions

Kurt Roeckx kurt at roeckx.be
Mon Feb 19 10:18:54 PST 2007


On Mon, Feb 19, 2007 at 07:35:40AM -0700, Rob Seaman wrote:
> On Feb 18, 2007, at 10:32 PM, David L. Mills wrote:
> 
> >There is no such thing as accuracy in the product, only a  
> >statistical estimation of the resolution, precision and other  
> >statistics. There is no specific accuracy statement possible in  
> >this thing. It's like saying, how pretty is your girlfriend.
> 
> Isn't the same issue as traceability?  Perhaps convolved with the non- 
> parametric nature of the clock discipline?  The whole point of  
> timekeeping is to tie a remote clock to a master time standard.  This  
> is distinct, say, from identifying physical limits on the accuracy of  
> absolute intervals returned by an egg timer.
> 
> In general, unless a trusted measurement of whatever quantity is  
> available, an estimate of accuracy is impossible.  The mental picture  
> usually used to convey the difference between accuracy and precision  
> is target practice.  Tight clustering of bullet holes is precision.   
> Having the average pattern spread coincide with the center of the  
> target is accuracy.

The maximum error in a measurement is the sum of the error caused by
accuracy and precision.  What we do is calculate the maximum error (and
estimate the error).  From this number you can't split it up in which
part is caused by the accuracy and which by the precision error.

If we could somehow calculate the accuracy error, and the resolution of
our clock is smaller than that, we could reduce it to something in the
order of our resolution.

Anyway, depending on the application, you're either interrested in the
maximum total error or in the precision.  I can't think of something
were you're interrested in the accuracy.


Kurt



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