Re: [PSAMP] PSAMP-INFO IE realtiveError

["Zseby, Tanja" <[e-mail]>]

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%

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. 

Hope this was not even more confusing...

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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