[Stoves] WBT disagrements

[Crispin Pemberton-Pigott]

"So the mass display to 1 gram is the accuracy and the +-2g is the precision"

The other way round. 

The display reads to 1 g but it might be slightly wrong. The result is guaranteed to be correct within 2 g, assuming of course it was calibrated. 

Adding another digit to read 0.1 g would not improve ‎the accuracy which an inherent property of the mass measurement system. Reporting the value to the indicated 0.1 would be 'false precision' which is true but a bit confusing, because it is implying 'false accuracy'. 

The bottom line is that all measurements should be reported with the uncertainty alongside.  

The result in the two examples above could be 123 g ±2 g, or 123.3 ±2 g. Both are correct and not really different. Reporting 123.2123 ±2 g is not really more helpful. 

Except under one condition: making a large number measurements (as the Adam scale can provide) allows one to report more digits of precision, giving not a 'more accurate answer', but an answer with a better known position of the centre of the uncertainty range. 

A good example of this are claims for the global average temperature to be 0.001 degree higher in 2016 than in 2015. The calculated average may indeed have given 0.001 as the value, but the uncertainty is more than ±0.5 degrees so the claim for 'hotter' is not supported, with confidence, by the evidence. 

There is a formula for how much additional precision you can report based on the number of readings, but that doesn't change the accuracy which remains ±2 g, if that is the rated performance. 

The Adam and Mettler-Toledo heads can both be made to report additional digits to have a more precise but not guaranteed number, and lots of these can be used to get a better answer over a measurement interval than a single number grabbed at some arbitrary time. 

Interestingly, the modern scale heads are built with several functions that make this achievement more difficult!

Regards
Crispin 


On 19 January 2018 at 00:33, Crispin Pemberton-Pigott
<crispinpigott at outlook.com> wrote:

>
> If the display shows mass by one gram and the 'guarantee' of the value shown is ±2 g, you have the rating of your scale.

So the mass display to 1 gram is the accuracy and the +-2g is the precision

Andrew

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