[LEAPSECS] Accuracy and Precision

[Michael Deckers]

    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.