On 2011-01-12 17:33, Finkleman, Dave wrote:
These terms [accuracy and precision] have appeared in recent exchanges. Keeping the distinction clear is one of my continuing quests. Perhaps I have it wrong, too. I am sure that someone will let me know. Accuracy is how well a measurement compares to a standard. If my one meter measuring stick is not one meter long, every measurement I make with it will be inaccurate. Precision is the variation among measurements. Even if the measuring stick is absolutely one meter long, every time I make a measurement, I may misplace it a bit. Each realization of the same measurement will be different.
Good! These definitions are in fact close to the ones in VIM (International vocabulary of metrology, by the Joint Committee for Guides in Metrology, headed by the BIPM and online at [www.bipm.org/en/publications/guides/vim.html]).
UTC provides precise time intervals for most practical purposes. However, it is inaccurate as the difference between UTC and time scales based on Earth rotation grows. I know precisely at the end of 86,400 SI seconds, that my perception of where I am in space is wrong.
I do not understand: accuracy applies to any measurement, precision applies to repeated measurements of a single measurand. If UTC is considered as the (combined) result of measurements (to which these notions would be applicable) then what is the measurand? Shouldn't UTC be considered as a physical quantity that can be measured in diverse ways (NTP, GPS, UTC(k),..), each method having its own accuracy and precision? Or do you allude to the fact that UTC has a non-zero definitional uncertainty (as does TAI) -- which a fortiori is a bound for the accuracy of any measurement of UTC? Finally, if I want to know where I am after 86 400 s, I would use an inertial coordinate system for time and space -- the "rotating geoid" with UTC or TAI is not (part of) one of those. Michael Deckers. _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
