Re: Media Testing for maclife.de
Re: Media Testing for maclife.de
- Subject: Re: Media Testing for maclife.de
- From: Chris Cox <email@hidden>
- Date: Thu, 18 Sep 2008 12:12:30 -0700
- Thread-topic: Media Testing for maclife.de
On 9/17/08 7:53 PM, "David Scharf" <email@hidden> wrote:
> Chris Cox wrote:
>> On 9/16/08 8:11 PM, "David Scharf" <email@hidden>
>> <mailto:email@hidden> wrote:
>>>
>> One of the instruments does not match standards which means you don¹t know
>> the correct scaling for the instrument. Both measurements are valid (barring
>> someone reading you random numbers). You just don¹t know how to interpret
>> them to match your standard. And you define ³correct² as meaning ³matches
>> the standard². If you find the mapping from instrument 2 to the standard
>> you can still use the measurements. Instrument 1 just had it¹s mapping done
>> ahead of time (which you can¹t do with all measurement devices,
>> unfortunately).
>>
> "Don't know the correct scaling"!!?? What? The premise is that we did know the
> correct scaling. By "scaling", I mean calibration, perhaps you mean mapping of
> the instrument's mis-calibration. In the practice of making precision
> measurements, scientists and engineers rely on properly calibrated equipment
> as a "first line of defense". We do not make software "profiles" of poorly
> calibrated instruments in order to make them usable. We just calibrate them
> against known precision standards.
>>
>> Again, the measurement itself is just a number (or set of numbers) - by
>> itself, it cannot be wrong.
>>
> Well, yes it can be wrong! Actually if you take the opposite attitude to
> yours, one could say that most almost measurements are incorrect (wrong). Its
> just a matter of how incorrect (imprecise). Although I originally thought you
> had just mis-spoke, apparently you can be wrong too. I am surprised at your
> attitude about it.
>>
>> It is an important point in sciences: just because the measurement doesn¹t
>> match what you expected, that does not make the measurement wrong. Usually
>> it just means that there is something you failed to account for in the theory
>> that predicted the values. If we threw away all results that didn¹t match
>> expectations, much of our scientific discovery would never happen.
>>
>>
>> Chris
> Well Chris, its good that you work for a software company and don't need to do
> lots of test and measurement work. I suppose that everything can be corrected
> in software now, eh? 8-)
>
David, you¹re just confusing things further. Apparently you¹re missing the
concept (I think you are just mixing the measurement with the
interpretation) , and always trust calibration to always be perfect. And
your insults are completely misdirected.
Yes, things are much easier if you assume perfect calibration then your
measurement and interpretation should always match. And if you¹re doing
clinical work (repeating a procedure, not trying to define new experiments)
it might be ok. But in the real world, and certainly in experimental
sciences you don¹t always have perfect calibration. The closer you get to
the cutting edge of research, the worse it gets. Scientists have been doing
post-measurement calibration for as long as science has existed.
Again: a measurement itself cannot be wrong. The interpretation of the
measurement can disagree with a standard or another measurement. And that
just means that you either didn¹t understand the experiment or the
instruments correctly.
I already gave you examples of perfectly valid measurements that did not
appear to agree at first. The temperature example could have been 3 readings
of boiling water on different scales, or could just as easily have been 3
readings from instruments with the same calibration inside a broken kiln.
The measurements are just numbers, they only appear to disagree if you
assign the same meaning to all the numbers. But the meaning depends on the
instruments, and the experimental setup. This is a very, very fundamental
concept in experimental sciences.
Chris
PS. The calibration of your instruments is defining a scaling and offset, or
calibration curve that can be done in the instrument, or outside. And
many modern instruments do use software to perform the calibration.
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