[LEAPSECS] Accuracy and Precision

Michael Deckers michael.deckers at yahoo.com
Wed Jan 12 14:19:57 EST 2011



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.


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