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Precise nature

Letters

On their data sheets for geotechnical instrumentation, some manufacturers give a number for resolution, but nothing for accuracy or repeatability.

I don't find this useful. It can be misleading if a user is unsure of the meaning of the terms, and believes that this number refers to accuracy or precision.

Resolution is the smallest division on the instrument readout scale.Accuracy is the closeness of approach of a measurement to the true value of the quantity measured.

Precision (a synonym for repeatability) is the closeness of approach of each of a number of similar measurements to the arithmetic mean.

As a user, I need to know either accuracy or precision. I need to know about accuracy when I'm interested in an absolute measurement, such as when measuring pore water pressure or total stress.

I need to know about precision when I'm interested in a measurement of change, such as when measuring deformation (I don't care about actual position in this case, only about change of actual position).Do I care about resolution? I don't think so - it seems to me that this is one of the issues that a manufacturer needs to address in system design, in order to arrive at the stated accuracy or precision.

If this is correct, please tell us about accuracy or precision, and keep the resolution issue to yourselves. If you have a different view, please share it with me.

On a similar subject, I recently found that an instrument didn't live up to the manufacturer's stated accuracy.We discussed it.The manufacturer offered, in subsequent publicity, to 'reset user expectations'.

I thought that was a masterpiece of gobbledegook.

John Dunnicliff, geotechnical instrumentation consultant, Devon

This letter is an extract from the September 2000 issue of John Dunnicliff 's regular Geotechnical Instrumentation News (GIN) column written for Geotechnical News, and is used here with the kind permission of BiTech Publications, Richmond, British Columbia, Canada.

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