Re: [PSAMP] PSAMP-INFO IE realtiveError

[Andrew Johnson <[e-mail]>]

Hi Tanja


On Sat, 2008-08-09 at 01:19 +0200, Zseby, Tanja wrote:
> Hi Andrew,
> 
> > -----Original Message-----
> > From: [e-mail] [mailto:[e-mail]] On Behalf
> > Of Andrew Johnson
> > Sent: Friday, August 08, 2008 8:48 PM
> > To: Zseby, Tanja
> > Cc: psamp; Juergen Quittek
> > Subject: Re: [PSAMP] PSAMP-INFO IE realtiveError
> > 
> > 
> > On Fri, 2008-08-08 at 18:07 +0200, Zseby, Tanja wrote:
> > > Hi Andrew,
> > >
> > > lets first forget about the fixed error and say we agree that we
> need
> > > something like an absolute error that defines the maximum error that
> > > can happen at each measurement (given the real error is unknown).
> > > Then it was unclear for me why you report this together with a
> > > relative error which provides exactly the same information but only
> > as
> > > percentage of the measured value. It is only for convenience that we
> > > can report either format.
> > >
> > > relError=abserror/measured value
> > >
> > >
> > > e.g. you can e.g. say: The absolute error is +/- 0.2 kg:
> > > Person:   80.50kg +/- 0.2kg
> > > Mouse:     0.50 kg +/- 0.2kg
> > >
> > > That corresponds to the relative errors:
> > > Person:   0.249 %
> > > Mouse:   40%
> > 
> > In the Accuracy Report Interpretation you only provide one accuracy
> for
> > the field, you don't report the accuracy per measurement.  The idea is
> > to provide the margin of error for all measurements of a certain type.
> > 
> > The only use of the error field types that was originally intended was
> > in the Accuracy Report Interpretation, where the error is scoped to
> the
> > field (and optionally template).  Unless you use a new template per
> > record I'm not sure how you would scope the error value to an
> > individual measurement.
> 
> But this is absolutely in-line with the above. You can provide one
> absolute error for all timestamps or all byte measurements (or all
> weight measurements). With this you say what is the maximum error when
> measuring the timestamp or bytes. This maximum error usually depends on
> the measurement method and therefore the absolute error (= the maximum
> possible error) is usually the same for all of the values measured with
> this method. I think this is exactly the same that you want to express,
> correct? 

Ah I see what you mean now.  In general I think that only one type of
error will be reported at once and that most of the time that will be an
absolute error.

I think there is still value in providing a way to express a relative
error though.  For example, clocks tend to drift over time so the larger
the time measurement the larger the error, i.e. accurate to within 1
second per day is 0.0011574%.

Cheers

Andrew


> > 
> > 
> > > Or you could say: The relative error is +/- 10 %. Then you get the
> > > corresponding absolute errors:
> > > Person:   80.50kg +/- 8.05 kg
> > > Mouse:     0.50 kg +/ 0.05 kg
> > >
> > > If this is o.k., the second question would be:  do we need something
> > > like an offset/fixed error ?
> > > e.g. Offset: 0.25
> > > Person (real value):   80.50kg
> > > Person (measured):     80.75kg
> > >
> > > Mouse (real value):   0.50 kg
> > > Mouse (measured):     0.75 kg
> > >
> > > The only thing that might be confusing is if you have an offset and
> > > report it together with a relative error, since the  relative error
> > > should still refer to the real value (without offset). But we
> > probably
> > > do not need the offset value.
> > 
> > I don't think we have any need for an offset.
> 
> o.k. I agree.
> 
> > 
> > > Hope this was not even more confusing...
> > 
> > I think I understand how you may have misunderstood how the error IEs
> > were intended to be used.  I hope I'm making things clearer... perhaps
> > the draft needs some clarification.
> 
> I think we have the same understanding (see above). The absoluteError
> gives the maximum value from which a measured value can differ from the
> real value. The error is usually bound to the measurement method or
> system (i.e. the same for subsequent values).
> 
> Kind regards,
> Tanja
> 
> > 
> > Cheers
> > 
> > Andrew
> > 
> > 
> > > Kind regards
> > > Tanja (starting to see white mice)
> > >
> > >
> > > > -----Original Message-----
> > > > From: Andrew Johnson [mailto:[e-mail]]
> > > > Sent: Friday, August 08, 2008 5:14 PM
> > > > To: Zseby, Tanja
> > > > Cc: Paul Aitken; psamp; Juergen Quittek
> > > > Subject: Re: [PSAMP] PSAMP-INFO IE realtiveError
> > > >
> > > > [SNIP]
> > > > > > >> The intention was to say that the clock is 5 minutes slow,
> > > > > > >> +/-
> > > > 10
> > > > > > >> seconds - so there's both an absolute error and a relative
> > > > error.
> > > > > > >>
> > > > > > >
> > > > > > > NOW I finally understand what you meant by fixedError
> > > > > > > initially
> > > > !!
> > > > > > > This I would not consider as error. Any fixed deviation I
> > > > > > > would
> > > > > > rather
> > > > > > > name "offset"...
> > > > > > > If it is known couldn't you add it to the value and report
> > the
> > > > > > correct
> > > > > > > time?
> > > > > >
> > > > > > In that case there would be no need to ever report
> > "absoluteError"
> > > > -
> > > > > > because all the original measurements can be corrected before
> > > being
> > > > > > exported.
> > > > >
> > > > > Maybe for clarification:
> > > > > The absoluteError that I propose is different from what you
> > > > > intended by fixedError. absoluteError is a maximum error that
> you
> > > > > would
> > > expect
> > > > > due to the inaccurate measurement (e.g. the timestamp may vary
> by
> > > +/-
> > > > 5 ms).
> > > > > The real error that you make during measurements is unknown and
> > > > > can vary. Your fixedError is different. It is a fixed and known
> > > > > offset
> > > > for
> > > > > the measured values, correct?
> > > >
> > > > I think the absoluteError is the same as the originally
> fixedError.
> > > In
> > > > Paul's example above the fixedError was +/- 10 seconds.  I'm not
> > > > sure how you would communicate the "5 minutes slow" part...
> > > >
> > > > The original idea was fixedError would say this is accurate to
> > > > within
> > > X
> > > > units.  Both the fixed and the absolute error can be used
> together,
> > > but
> > > > you just have to go with the least accurate one.  For example, if
> > my
> > > > bathroom scales have a fixed error of 0.25kg and a relative error
> > of
> > > > 0.5%, then they can weigh a person very accurate, but are rubbish
> > > > for weighing mice:
> > > >   Person1:   81.50kg +/- 0.4kg
> > > >   Mouse1:     0.25kg +/ 0.25kg
> > > >
> > > > Cheers
> > > >
> > > > Andrew
> > >
> 

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