Re: [PSAMP] PSAMP-INFO IE realtiveError

[Andrew Johnson <[e-mail]>]

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.


> 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.


> 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.

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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