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It seems we have two concepts that land surveyors like to call 
uncertainty a.k.a error, which is
acceptable because it is inherent in the nature of the measurement 
process, versus blunder
which is an error that would not have occurred if all parties taken 
proper care. So we might say
that, having first agreed to allow negative years, Julius Caesar died 
-43-03-1b. The date is unknown
not because of any uncertainty on the part of the assassins about the 
date, but to subsequent
blunders in observing leap years and keeping records as to when leap 
years were observed.

Gerry Ashton

On 8/2/2011 2:52 PM, Ray Denenberg, Library of Congress wrote:
> Ok, I think I have a solution to all of this.
>
> 'u' clearly has a much different usage when used internally than when it
> replaces one or more consecutive rightmost digits. I don't think we have yet
> confronted this.
>
> Thus 196u means:
>      "a date in the 1960s"
>
> and expresses imprecision, where 1u66 might be said to express uncertainty.
>
> In other words I suspect the solution to this problem is not to refer to
> "rightmost" 'u' as "uncertainly" and instead refer to it as "imprecision".
>
> And since 'u' means different things when used internally vs. rightmost, and
> perhaps 'u' shouldn't be used for both.
>
> I therefore propose that 'x' be used instead of 'u' for level 1 uncertainty,
> and that we rename "uncertainly" as "imprecision" for level 1 (and continue
> to call it 'uncertainty' for level 2 - internal).
>
> --Ray
>
>