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Resolution, Discrimination, NDC and the Tower of Babel in Metrology

D

dfirka

#1
Resolution,Discrimination,NDC and the Tower of Babel in Metrology

Dear Sirs

My (poor) understanding regarding these concepts are:

Resolution=Discrimination : smallest readable unit of an instrument. In the case of digital meters, it is equal to one unit of the least significant digit.

... but: in Mettler/Toledo balances, resolution is defined as the ratio between the maximum load and the readability (calling readability to the least significant digit).

NDC = Number of distinct categories that can reliably distinguished by the measurement system. Close to the Discrimination Ratio (DR) proposed by Wheeler(1989).

... but: reading The Six Sigma Handbook (Pyzdec,2003) (p.326) the author defines "resolution=discrimination" as the ability to divide measurements into "data categories", and his explanation is closer to the definition of ndc.

Is there an agreed consensus over resolution-discrimination and ndc? Or we are facing the confusion that already diminished the usability of "accuracy" and "precision".


Thxs
Daniel
 
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Hershal

Metrologist-Auditor
Staff member
Super Moderator
#2
You have some good questions.

May i suggest referring to VIM (International Vocabulary of General and Specific Terms in Metrology), and to the Metrology Handbook which is just coming out from ASQ?

Hope this helps.

Hershal
 

Wes Bucey

Prophet of Profit
#3
Hershal said:
You have some good questions.

May i suggest referring to VIM (International Vocabulary of General and Specific Terms in Metrology), and to the Metrology Handbook which is just coming out from ASQ?

Hope this helps.

Hershal
:bigwave: Welcome to the Cove, Mr. Brewer! Nice to see a familiar ASQ stalwart. I'm so happy you got a Metrology Forum at ASQ. Now if they would only appoint you the moderator!

Additional to Daniel and others:
ASQ books can be browsed and ordered here:
http://qualitypress.asq.org/
 
D

dfirka

#4
Hershal said:
May i suggest referring to VIM (International Vocabulary of General and Specific Terms in Metrology), and to the Metrology Handbook which is just coming out from ASQ?
Mr. Hershal

Thanks; I would also consider VIM definitions as the "highest echelon" regarding terminology.

Allow me to broaden my issue to a more general concern about the homogeneity/consistency of terms. Precisely defined terms are the basis for a "repeatable" communication process, and sometimes I find little observance of this paramount criterion.

Another example to illustrate this, following the subject of the thread: If we compare the definitions for resolution and discrimination between AIAG MSA Manual (2002) and the VIM:

Resolution
(VIM 5.12) : smallest difference between indications of a displaying device that can be meaningfully distinguished.
(AIAG) : capability of the measurement system to detect and faithfully indicate even small changes of the measured characteristic. The resolution of a measurement system is D if there is an equal probability that the indicated value of any part which differs from a reference part by less than D will be the same as the indicate value of the reference part. The resolution of a measurement system is impacted by the measurement instrument as well as other sources of variation of the total measurement system.

Discrimination
(VIM 5.11) : largest change in a stimulus that produces no detectable change in the response of a measuring instrument, the change in the stimulus taking place slowly and monotonically
(AIAG) : (Alias smallest readable unit) discrimination is the measurement resolution, scale limit, or smallest detectable unit of the measurement device and standard.

Is the AIAG transposing the definitions?
The increasing popularity of the MSA manual of AIAG (2002) can lead to "de facto" definitions that eventually can dilute the value of the term.

Daniel
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
Daniel,

You present an interesting situation there.

It seems that for resolution, AIAG is taking a long-winded approach to say the same thing as VIM.

However, for discrimination, it appears that AIAG is talking about the smallest measureable quantity that a device can reliably read, while VIM is talking about the largest change in the quantity being read that can occur without stimulating a change in the instrument(s). In this case, they also appear to be the same, except that VIM has not specified a limit on quantity, so the quantity can be towards the upper end of the instrument(s) capability and still be within the VIM definition, while being far outside the AIAG definition.

My guess is that this one will take awhile to shake out.....

Hershal
 
D

dfirka

#6
Hershal,

Interesting point. And delving a little bit deeper in this “resolution/discrimination” issue:

Regarding resolution, VIM clearly refers to a property of a displaying device, whereas AIAG extends the concept with two derived terms:
- "apparent resolution": The size of the least increment on the measurement instrument.
- "effective resolution": The size of the data category when the total measurement system (MS) variation is considered.

AIAG is interested in the capability of the MS to measure specific processes or products, and the “effective resolution” is subsequently used to detect the number of meaningful distinct categories into which a particular process could be segregated.

The “effective resolution” is quantitatively defined as the length of the confidence interval based on the measurement system variation, and here is where I find a departure from the original (VIM) definition of resolution.

As of discrimination, AIAG uses the term as a qualitative counterpart of “effective resolution” (i.e.: being acceptable or unacceptable for a certain application of a MS). In that sense, the VIM definition is “agnostic” to specific intended uses.

Maybe a rhetorical question: Wouldn’t be more appropriate to let “resolution” be a property of a displaying device, and widen the definition of “discrimination” to account for the MS variation and “segregation capability for a particular application”?

D. Firka
 
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